Micro-groove assisted ceramic to metal brazing method

Abstract

The application provides a micro-groove assisted ceramic and metal brazing method, a plurality of grooves are prepared on the surface of a ceramic substrate to be brazed, the grooves are strip-shaped and arranged periodically; then filler metal is laid between the ceramic substrate and a metal sheet to form a brazing assembly, and pressure treatment is performed; finally, vacuum brazing is performed on the brazing assembly. The ceramic and metal brazing method is particularly suitable for brazing between Si3N4 ceramic and Cu, and through the synergistic effect of mechanical interlocking of laser micro-grooves, interface reaction of close fitting and clean brazing atmosphere, the shear strength of the joint between Si3N4 ceramic and Cu is greater than or equal to 67 MPa, which is more than 2.5 times of that of a traditional process, and the thermal cycle stability meets the long-term service requirement of an IGBT module in the field of aerospace.

Description

Technical Field

[0001] This invention belongs to the field of ceramic / metal brazing technology, specifically relating to a microgroove-assisted ceramic-metal brazing method. Background Technology

[0002] In the third-generation semiconductor industry, IGBT (Insulated Gate Bipolar Transistor) modules, as core devices for power conversion and control, are widely used in strategic industries such as aerospace, rail transportation, and smart grids. Especially in the aerospace field, IGBT modules within spacecraft power systems and propulsion control systems need to operate stably for extended periods under extreme temperature fluctuations of -40℃ to 150℃ and strong mechanical vibrations. The performance of their core load-bearing component, the "copper-clad ceramic substrate," directly determines the lifespan and operational safety of the IGBT module.

[0003] Si3N4 ceramic, with its excellent comprehensive properties including high thermal conductivity, high-temperature strength, high thermal shock resistance, and low coefficient of thermal expansion, has become the preferred material for ceramic substrates of aerospace IGBT modules. Cu, with its high electrical and thermal conductivity, is an ideal metal conductive / heat dissipation layer for copper-clad ceramic substrates. Active metal brazing (AMB) is the mainstream process for joining Si3N4 ceramic and Cu. Its principle is to utilize the active elements such as Ti in the brazing filler metal to chemically react with Si3N4 ceramic to form an interfacial reaction layer, achieving metallurgical bonding. However, Si3N4 ceramic is a covalently bond-dominated non-oxide ceramic with strong surface chemical inertness and extremely poor wettability with Cu (contact angle >120°). Traditional brazing processes suffer from insufficient interfacial bonding strength (typically ≤45MPa), making it difficult to achieve a reliable connection and withstand the vibration and shock of aerospace environments.

[0004] In the prior art, the patent with publication number CN115557798A, "An AlN Ceramic Copper-Clad Substrate with Strong Bond to Copper Layer and Ceramic Substrate and Its Preparation Method," describes a process where AlN ceramic is first immersed in an activated precursor solution containing Ag, Pd, Ni, or / and Cu metal ions or metal complex ions. A microstructure is then obtained through laser etching, followed by laser activation to generate heterogeneous or / and homogeneous activated seed layers. This is then combined with an active metal brazing (AMB) copper cladding process to achieve a high bonding strength between the AlN ceramic and the copper foil. However, this process requires the pre-placement of an activated precursor solution containing noble metals such as Ag and Pd to form a seed layer, making it complex and costly. Due to the significant differences in the interfacial reaction mechanisms between AlN and Si3N4 (AlN surfaces readily form an Al2O3 transition layer, while Si3N4 requires an active Ti decomposition reaction), this technology cannot be directly applied to the bonding scenarios between Si3N4 and Cu. The patent with publication number CN113953609A, entitled "A Method for AMB Ceramic-Metal Brazing," employs a sandblasting + cold spraying process. This involves significant equipment investment and a cumbersome process, and the irregular, rough surface formed by sandblasting makes it difficult to achieve precise mechanical interlocking. Furthermore, existing laser-modified Si3N4 / Cu brazing technologies only focus on optimizing parameters such as laser power and scanning speed, resulting in limited improvement in joint strength and insufficient stability. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a microgroove-assisted ceramic-metal brazing method, solving the technical problems of insufficient joint strength and numerous defects in traditional brazing processes.

[0006] The present invention achieves the above-mentioned technical objectives through the following technical means.

[0007] A microgroove-assisted ceramic-metal brazing method includes the following steps:

[0008] Microgroove fabrication: Several grooves are fabricated on the surface of the ceramic substrate to be brazed, and the grooves are elongated.

[0009] Brazing assembly: A brazing filler metal is laid between a ceramic substrate and a metal sheet to form a brazing assembly;

[0010] Vacuum brazing: Vacuum brazing is performed on the brazing assembly.

[0011] Furthermore, the grooves are arranged periodically.

[0012] Furthermore, the method for preparing the microgroove is as follows: using a nanosecond pulsed laser, under a flowing argon protective atmosphere, to scan the surface of the ceramic substrate in a parallel path.

[0013] Furthermore, the parameters of the nanosecond pulsed laser include: wavelength 1064nm, repetition frequency 200~300kHz, pulse width 30~50ns, focused spot diameter 25~35μm, power 25~35W, scanning speed 400~600mm / s, and scanning spacing 40~100μm.

[0014] Furthermore, the scanning interval of the nanosecond pulsed laser is 60 μm.

[0015] Furthermore, for brazing Si3N4 ceramics to Cu, the brazing filler metal is an Ag-Cu-Ti active brazing filler metal with a Ti content of 4~5 wt.% and a thickness of 50~100 μm;

[0016] Before vacuum brazing, the brazing assembly is pressurized from both sides inwards, with a pressure range of 0.05~0.3MPa.

[0017] Furthermore, a tube furnace is used for vacuum brazing. Before brazing, the furnace is deoxygenated by circulating vacuum and argon gas for 3 to 8 cycles. After deoxygenation, vacuum brazing is performed by step heating.

[0018] Furthermore, the vacuum brazing process includes:

[0019] Stage 1: Increase the temperature to 300-400℃ at a rate of 8-15℃ / min, and hold for 10-20 minutes;

[0020] Stage 2: Continue to heat at the same rate to 820~860℃ and hold for 30~40 minutes;

[0021] Phase 3: After the heat preservation is completed, control the cooling rate inside the furnace to 4~6℃ / min, and cool the furnace to room temperature.

[0022] Furthermore, the heating rate of stage 1 and stage 2 is 10~12℃ / min.

[0023] Furthermore, the following pretreatment is performed on the ceramic and metal substrates to be brazed:

[0024] Grinding: Grinding was performed sequentially using 1000-mesh, 1500-mesh, and 2000-mesh SiC sandpaper;

[0025] Cleaning: After grinding, place the substrate in anhydrous ethanol for ultrasonic cleaning for 15-20 minutes;

[0026] Drying: After cleaning, dry the substrate.

[0027] The beneficial effects of this invention are as follows:

[0028] (1) This invention provides a microgroove-assisted ceramic-metal brazing method, which is particularly suitable for brazing between Si3N4 ceramic and Cu. Through the synergistic effect of mechanical interlocking of laser microgroove, tight interface reaction, and clean brazing atmosphere, the joint shear strength of Si3N4 ceramic and Cu is ≥67MPa, which is more than 2.5 times that of traditional process. The thermal cycling stability meets the long-term service requirements of IGBT modules in the aerospace field.

[0029] (2) By applying pressure to the brazing assembly, the present invention can ensure that the brazing filler metal is in close contact with the substrates on both sides, while avoiding excessive pressure that could cause Si3N4 ceramic to crack.

[0030] (3) Before vacuum brazing, the present invention deoxygenates the furnace by repeatedly evacuating and filling with argon gas to avoid oxidation of Ti elements at the effective bonding site to form a brittle TiOx phase.

[0031] (4) In the vacuum brazing process of the present invention, the temperature is raised in a stepwise manner. The first stage of heating is used to remove residual water vapor and ethanol on the surface of the substrate and reduce the temperature gradient inside and outside the component. The heating rate is controlled at 10~12℃ / min, which can balance the heating efficiency and temperature gradient to the greatest extent and avoid the substrate from cracking due to thermal stress.

[0032] (5) The process of this invention is simple and controllable, requiring no precious metal precursors, special spraying equipment or complex tooling. Laser parameters, pressure, and brazing atmosphere can all be precisely controlled, making it suitable for mass production. It can also meet the requirements of "low maintenance, long life and lightweight" for spacecraft, and can be extended to the connection of other non-oxide ceramics such as SiC ceramics and ZrO2 ceramics with metals. The technology has strong versatility. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the equipment setup for the microgroove preparation step of the present invention;

[0034] Figure 2 This is a schematic diagram of the brazing assembly of the present invention;

[0035] Figure 3 This is a schematic diagram of the brazing of the tubular furnace of the present invention;

[0036] Figure 4 This is a microstructure diagram of the interface of the brazed joint prepared in the embodiments of this application;

[0037] Figure 5 This is a comparison diagram of the joint shear strength under different laser scanning intervals in this invention.

[0038] Figure label:

[0039] 1-Laser beam; 2-Glass cover; 3-Ceramic substrate; 4-Worktable;

[0040] 5-Argon gas cylinder; 6-Support; 7-Brazing assembly; 8-Pure Cu sheet;

[0041] 9-Bracket filler metal; 10-Buffer pad; 11-Clamp; 12-Tube furnace;

[0042] 13-Vacuum pump; 14-Reaction layer. Detailed Implementation

[0043] Embodiments of the present invention are described in detail below. Examples of the embodiments are shown in the accompanying drawings, wherein similar or identical reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0044] I. Technical Solution

[0045] For brazing Si3N4 ceramics to Cu, this invention provides the following brazing method based on nanosecond laser microgroove assistance:

[0046] 1. Substrate pretreatment

[0047] For the Si3N4 ceramic substrate and pure Cu sheet to be welded, they are ground and polished with 1000-mesh, 1500-mesh and 2000-mesh SiC sandpaper respectively to remove the oxide layer and scratches on the surface of both. The ground substrate is placed in anhydrous ethanol for ultrasonic cleaning for 15-20 minutes to remove surface oil and particulate impurities, and then dried for later use.

[0048] 2. Microgroove preparation

[0049] like Figure 1 As shown, a nanosecond pulsed laser system was used to scan the surface of a Si3N4 ceramic substrate along a parallel path under a flowing argon protective atmosphere, thereby fabricating a periodically arranged microgroove structure on it. Figure 1 The microgroove fabrication equipment shown includes: a worktable 4 and an argon gas cylinder 4 mounted on a support 6 outside the worktable 4; a glass cover 2 is mounted on the worktable 4, and a Si3N4 ceramic substrate 3 is placed inside the glass cover 2; the argon gas cylinder 4 is connected to the inner cavity of the glass cover 2 through a gas pipe, and is used to fill the glass cover 2 with argon gas to provide argon protection for the Si3N4 ceramic substrate 3; a laser beam 1 penetrates the glass cover 2 and is directed towards the Si3N4 ceramic substrate 3 to fabricate microgrooves.

[0050] The laser parameters were set as follows: wavelength 1064nm, repetition rate 200~300kHz, pulse width 30~50ns, focused spot diameter 25~35μm, power 25~35W, scanning speed 400~600mm / s, and scanning spacing 40~100μm. Figure 5As shown, the scanning spacing is the core control parameter of this invention. When the spacing is 60μm, the prepared microgroove forms a trapezoidal cross section (depth 26μm, top width 20μm), which increases the contact area with the solder by more than 50% compared to the original smooth surface. This allows for optimal mechanical interlocking with the Ag-Cu-Ti solder, and the joint shear strength between Si3N4 and Cu is the highest.

[0051] During the laser processing described above, the Si3N4 ceramic surface undergoes thermal decomposition after absorbing laser energy. The generated silicon atoms are deposited on the trench surface, forming a recast silicon layer with a thickness of 50~100nm, which can significantly improve the chemical activity of the ceramic surface.

[0052] 3. Brazing assembly

[0053] like Figure 2 As shown, an Ag-Cu-Ti active solder with a Ti content of 4~5 wt.% is used, and the thickness is controlled to be 50~100 μm. The solder 9 is uniformly laid between the Si3N4 ceramic substrate 3 treated above and the pure Cu sheet 9 to form a "Si3N4 ceramic-solder-Cu" brazing assembly 7.

[0054] A stainless steel metal clip 11 (temperature resistance >800℃) is used, and pressure is applied to it from both sides of the brazing assembly 7. The pressure range is controlled within 0.05~0.3MPa (preferably 0.1~0.15MPa). This ensures that the brazing filler metal is in close contact with the substrate on both sides, while avoiding excessive pressure that could cause the Si3N4 ceramic to crack. A 1mm thick high-temperature resistant alumina buffer pad 10 can be attached to the contact area between the clip 11 and the brazing assembly 7 to prevent direct contact between the clip 11 and the substrate surface from causing damage.

[0055] 4. Vacuum brazing

[0056] like Figure 3 As shown, the pressurized brazing assembly 7 is placed into the vacuum tube furnace 12. Using a vacuum pump 13 and an argon cylinder 5, the tube furnace 12 is evacuated and then purged with argon gas to remove oxygen. Specifically: first, the furnace is evacuated to 10... 5 After reaching a pressure of ~10²Pa, the vacuum pump 13 is turned off and high-purity argon gas with a purity ≥99.99% is introduced to atmospheric pressure; the above operation is repeated 3 to 8 times. Typically, after 5 cycles, the residual oxygen content in the furnace can be reduced to below 50ppm, thereby preventing the oxidation of Ti elements at effective bonding sites to form the brittle TiOx phase.

[0057] After the above-mentioned deoxygenation cycle, the vacuum pump is turned on to continuously evacuate the tube furnace to ensure the vacuum level inside the furnace. Then, the tube furnace is started for stepwise heating.

[0058] In stage 1, the temperature is increased to 300-400℃ at a rate of 8-15℃ / min and held for 10-20min to remove residual moisture and ethanol on the substrate surface and reduce the temperature gradient inside and outside the component. The optimal heating rate is 10-12℃ / min, which can maximize the balance between heating efficiency and temperature gradient and avoid substrate cracking due to thermal stress.

[0059] In stage 2, the temperature is increased to 820~860℃ at the same rate and held for 30~40 minutes. Within this temperature range, the Ag-Cu-Ti solder melts completely. The Ti element preferentially reacts with the recast silicon layer on the surface of the Si3N4 microgroove to form Ti5Si3. The remaining Ti reacts with the N element decomposed from Si3N4 to form TiN, forming a continuous Ti5Si3+TiN interface reaction layer.

[0060] Phase 3: After the heat preservation is completed, turn off the heating device and control the cooling rate in the furnace to 4~6℃ / min. Cool the furnace to room temperature to reduce the thermal expansion difference stress between Si3N4 and Cu.

[0061] II. Testing and Verification

[0062] Example

[0063] Step 1: Commercially available Si3N4 ceramic and 99.99wt.% pure Cu sheets were selected and processed into 15mm×15mm×1mm dimensions. The surface was then sequentially polished with 1000-mesh, 1500-mesh, and 2000-mesh SiC sandpaper, followed by ultrasonic cleaning with anhydrous ethanol for 5 minutes after each polishing pass. After the final polishing, the surface was ultrasonically cleaned in anhydrous ethanol at 40kHz for 20 minutes and dried at 50℃. A nanosecond pulsed laser system (model: YFPN-100-GM-LR03071A) was used under argon gas flow protection at a parallel path to scan the Si3N4 surface: wavelength 1064nm, repetition frequency 260kHz, pulse width 45ns, focused spot diameter 30μm, power 30W, scanning speed 500mm / s, and scanning spacing 60μm. The resulting microgrooves had a trapezoidal cross-section, a depth of 26μm, a top width of 20μm, and were covered with a certain thickness of recast silicon layer.

[0064] Step 2: Select Ag-Cu-4.5Ti solder (Ti content 4.5wt.%, thickness 80μm) and evenly sandwich it between Si3N4 ceramic and Cu. Use 304 stainless steel clips to apply 0.1MPa pressure from both sides. Attach 1mm thick alumina buffer pads to the contact parts of the clips to ensure that the components fit together without gaps.

[0065] Step 3: Place the components into an AFD1200-60 vacuum tube furnace, start the mechanical pump to evacuate to 10³Pa, fill with 99.99% argon gas to atmospheric pressure, and repeat the operation 5 times; heat to 350℃ at a rate of 10℃ / min and hold for 15min; continue to heat to 840℃ at a rate of 10℃ / min and hold for 35min; turn off the heating device, control the cooling rate to 5℃ / min, and cool to room temperature with the furnace.

[0066] The performance tests for the brazed joints prepared by the above method are as follows:

[0067] (1) Shear strength: The DDL100 universal testing machine was used with a loading rate of 0.5 mm / min. The average value of the three tests was 67.7 MPa.

[0068] (2) Interface morphology: such as Figure 4 As shown, FESEM observation revealed that the solder in the microgroove was fully filled, and a continuous Ti5Si3+TiN reaction layer 14 (2.5 μm thick) was formed at the interface, without pores or cracks.

[0069] (3) Thermal cycling stability: After 500 cycles of -40℃ (30 min) to 150℃ (30 min), the joint shear strength retention rate is 90%, with no cracks or detachment.

[0070] Comparative Example 1 (without laser microgrooves)

[0071] Except for the absence of laser microgroove fabrication, the remaining steps were the same as in the previous embodiment. Test results: The joint shear strength was 25.2 MPa, there was no mechanical interlocking structure at the interface, the fracture mode was Si3N4 / solder metal interface fracture, and cracks appeared after 50 thermal cycles.

[0072] Comparative Example 2 (with laser microgrooves but without pressure)

[0073] Except for step 2 where no pressure was applied, the remaining steps were the same as in the embodiment. Test results: The joint shear strength was 35.6 MPa, there were tiny gaps at the interface, the brazing filler metal was not fully filled, and local cracks appeared after 200 thermal cycles.

[0074] Comparative Example 3 (with laser microgrooves but without vacuuming-argon purging cycle)

[0075] Except for step 3, which only involved one vacuuming cycle without deoxygenation via vacuuming and argon purging, the remaining steps were the same as in the previous embodiment. Test results: The brazed joints were not effectively bonded; the Si3N4 ceramic substrate and the Cu sheet separated; and a large amount of brittle TiOx phase was generated at the interface, affecting the effective bonding of the components.

[0076] In the description of this invention, it should be understood that the terms "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0077] This invention is not limited to the above-described embodiments. Any obvious improvements, substitutions, or modifications that can be made by those skilled in the art without departing from the essence of this invention are within the scope of protection of this invention.

Claims

1. A microgroove-assisted ceramic-metal brazing method, characterized in that: Includes the following steps: Microgroove fabrication: Several grooves are fabricated on the surface of the ceramic substrate to be brazed, and the grooves are elongated. Brazing assembly: A brazing filler metal is laid between a ceramic substrate and a metal sheet to form a brazing assembly; Vacuum brazing: Vacuum brazing is performed on the brazing assembly.

2. The microgroove-assisted ceramic-metal brazing method according to claim 1, characterized in that: The grooves are arranged periodically.

3. The microgroove-assisted ceramic-metal brazing method according to claim 2, characterized in that: The method for preparing the microgrooves is as follows: using a nanosecond pulsed laser, under a flowing argon protective atmosphere, to scan the surface of the ceramic substrate in a parallel path.

4. The microgroove-assisted ceramic-metal brazing method according to claim 3, characterized in that: The parameters of the nanosecond pulsed laser include: wavelength 1064nm, repetition frequency 200~300kHz, pulse width 30~50ns, focused spot diameter 25~35μm, power 25~35W, scanning speed 400~600mm / s, and scanning spacing 40~100μm.

5. The microgroove-assisted ceramic-metal brazing method according to claim 3, characterized in that: The scanning interval of the nanosecond pulsed laser is 60 μm.

6. The microgroove-assisted ceramic-metal brazing method according to claim 1, characterized in that: For brazing Si3N4 ceramics to Cu, the brazing filler metal is an Ag-Cu-Ti active brazing filler metal with a Ti content of 4~5wt.% and a thickness of 50~100μm; Before vacuum brazing, the brazing assembly is pressurized from both sides inwards, with a pressure range of 0.05~0.3MPa.

7. The microgroove-assisted ceramic-metal brazing method according to claim 1, characterized in that: Vacuum brazing is performed using a tube furnace. Before brazing, the furnace is deoxygenated by 3 to 8 cycles of vacuuming and argon filling. After deoxygenation, vacuum brazing is performed by stepwise heating.

8. The microgroove-assisted ceramic-metal brazing method according to claim 7, characterized in that: The vacuum brazing process includes: Stage 1: Increase the temperature to 300-400℃ at a rate of 8-15℃ / min, and hold for 10-20 minutes; Stage 2: Continue to heat at the same rate to 820~860℃ and hold for 30~40 minutes; Phase 3: After the heat preservation is completed, control the cooling rate inside the furnace to 4~6℃ / min, and cool the furnace to room temperature.

9. The microgroove-assisted ceramic-metal brazing method according to claim 8, characterized in that: The heating rate for stages 1 and 2 is 10~12℃ / min.

10. The microgroove-assisted ceramic-metal brazing method according to claim 1, characterized in that: For ceramic and metal substrates to be brazed, the following pretreatment is performed: Grinding: Grinding was performed sequentially using 1000-mesh, 1500-mesh, and 2000-mesh SiC sandpaper; Cleaning: After grinding, place the substrate in anhydrous ethanol for ultrasonic cleaning for 15-20 minutes; Drying: After cleaning, dry the substrate.

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