A warpage prevention method and special fixture for a double-sided copper clad ceramic substrate during sintering

Abstract

The application discloses a warping prevention method and a special jig in a double-sided copper-clad ceramic substrate sintering process, and comprises the following steps: step one, a first surface sintering process; step two, warping parameter detection; step three, a pre-pressing shape correction process; the primary sintered substrate is placed in the jig, the pre-pressing target warping degree W2 is set according to the detected warping parameters and material characteristics, directional pressure is applied to the copper sheet convex side of the primary sintered substrate through the pressing assembly of the jig, the substrate is reversely bent to the target warping degree and is kept under pressure, and part of the residual stress is eliminated; step four, a second surface sintering process; the pre-pressing state of the jig to the primary sintered substrate is kept, the substrate kept in the pre-pressing state is sent into a sintering furnace, and copper sheet sintering is carried out on the second surface of the ceramic substrate; in the sintering process, the temperature compensation assembly controls the temperature difference between the upper and lower surfaces of the substrate to be within ± 5 DEG C, and ensures that thermal stress is uniformly released; and step five, a cooling demolding process; precise correction of the warping deformation after single surface sintering is realized, the flatness of the substrate after secondary sintering is ensured, and the internal stress of the substrate is reduced.

Description

Technical Field

[0001] This invention relates to the field of copper-clad ceramic substrate (DBC substrate) manufacturing technology, specifically to a method and special fixture for preventing warping during the sintering process of double-sided copper-clad ceramic substrate. Background Technology

[0002] Copper-clad ceramic substrates (DBC substrates) are widely used in high-end fields such as power electronics, semiconductor lighting, and automotive electronics due to their excellent thermal conductivity, insulation, and mechanical stability. The core process in manufacturing double-sided copper-clad ceramic substrates is the sintering bonding of the copper sheets to the ceramic substrate. This requires sintering the copper sheets on both sides twice to form a strong metallurgical bond between the copper sheets and the ceramic substrate.

[0003] However, copper and ceramic materials have significantly different coefficients of thermal expansion (copper's coefficient of thermal expansion is approximately...). Alumina ceramics are approximately Aluminum nitride ceramics are approximately This mismatch in thermal expansion coefficients is a key factor causing warping deformation during substrate sintering. Especially in scenarios involving a combination of thick copper and thin ceramic (e.g., copper sheet thickness ≥ 0.3 mm, ceramic substrate thickness ≤ 0.38 mm), after the first copper sheet is sintered, the substrate will experience significant warping deformation due to the difference in thermal shrinkage between copper and ceramic, with warping typically exceeding 5 mm.

[0004] In the existing technology, there is a lack of effective solutions to the aforementioned warping problem: when directly sintering the second side, the warped substrate cannot fit well with the sintering fixture in the sintering furnace, resulting in uneven sintering temperature and insufficient bonding strength of the copper sheet on the second side. Furthermore, the warping deformation of the substrate cannot be corrected after the second sintering, and may even cause the substrate to crack due to stress superposition. Some production processes use simple heavy-weight pressure for shaping, but it is impossible to accurately control the pressure and warping correction degree, which can easily cause excessive local stress on the substrate and damage it. At the same time, it is impossible to ensure uniform stress distribution inside the substrate, and the substrate is prone to secondary warping due to stress release after sintering. Even with specialized pre-correction... The fixture also has significant drawbacks: this type of pre-correction fixture can only reduce the warpage height of the product to the set height, but it does not effectively correct the curvature of the single-sided copper-clad substrate after correction, resulting in uneven curvature or inconsistent height in different areas of the substrate, and the subsequent adhesion with the sintering fixture is still poor; moreover, the warpage reduction achieved by pressing is essentially a forced elastic deformation, and the product will elastically rebound after the pressure is released. More importantly, the material uniformity and initial warpage shape of different copper-clad boards are different, resulting in inconsistent rebound heights of each copper-clad board, which seriously affects product consistency and makes it difficult to guarantee the quality stability of the subsequent second-sided sintering.

[0005] Therefore, there is an urgent need for a technical solution that can precisely control the warping deformation of double-sided copper-clad ceramic substrates during sintering, and ensure the flatness and stress uniformity of the substrate. Summary of the Invention

[0006] To address the problems of excessive warping after single-sided sintering, poor flatness after secondary sintering, and uneven internal stress caused by the mismatch of thermal expansion coefficients between copper and ceramic and the "thick copper-thin ceramic" combination in the existing double-sided copper-clad ceramic substrate sintering process, this invention provides a method and special fixture for preventing warping during the double-sided copper-clad ceramic substrate sintering process. This method can accurately correct the warping deformation after single-sided sintering, ensure the flatness of the substrate after secondary sintering, and reduce the internal stress of the substrate.

[0007] The technical solution of this invention is: a method for preventing warping during the sintering process of double-sided copper-clad ceramic substrates, the specific steps of which are as follows:

[0008] Step 1: First surface sintering process;

[0009] The first side of the ceramic substrate is conventionally sintered with the copper sheet to obtain a single-sided copper-coated initial sintered substrate. At this time, the initial sintered substrate warps due to the difference in thermal expansion coefficients between copper and ceramic. The warping direction is that the copper sheet side is convex and the ceramic side is concave.

[0010] Step 2: Warp parameter detection;

[0011] The maximum warpage of the initial sintered substrate was detected using a laser warpage meter. Record the thickness of the ceramic substrate and the location of the warpage center. Copper sheet thickness and their coefficients of thermal expansion , ;

[0012] Step 3: Pre-pressing and shaping process;

[0013] The pre-sintered substrate is placed in the fixture, and the pre-compression target warpage is set according to the detected warpage parameters and material properties. The pressure component of the fixture applies directional pressure to the copper sheet protrusion side of the initial sintered substrate, causing the substrate to bend in the opposite direction to the target warp and hold the pressure to eliminate some residual stress.

[0014] Step 4: Second-side sintering process;

[0015] The fixture maintains the pre-pressed state of the initially sintered substrate, and the substrate in the pre-pressed state is sent into the sintering furnace for copper sheet sintering on the second side of the ceramic substrate. During the sintering process, the temperature compensation component controls the temperature difference between the upper and lower surfaces of the substrate within ±5℃ to ensure uniform release of thermal stress.

[0016] Step 5: Cooling and demolding process;

[0017] After sintering, the furnace is cooled to room temperature, and the substrate is removed, resulting in a flat double-sided copper-clad ceramic substrate with a final warpage. .

[0018] Furthermore, in step three, the target preload warpage is set. .

[0019] Furthermore, in step three, the substrate is bent in the reverse direction to the target warp degree and held under pressure for 5-10 minutes.

[0020] The present invention also provides a special fixture for implementing the anti-warping method in the sintering process of the double-sided copper-clad ceramic substrate, including a base, a positioning component, a pressurizing component, a temperature compensation component, and a control system;

[0021] The upper surface of the base is provided with a positioning groove that matches the shape of the substrate, and a shaping pad is laid at the bottom of the positioning groove;

[0022] The positioning component includes four adjustable positioning pins arranged around the positioning groove. The ends of the positioning pins are provided with elastic buffer pads. By adjusting the extension length of the positioning pins, accurate positioning of substrates of different sizes can be achieved, thus preventing substrate displacement during the calibration process.

[0023] The pressurizing assembly includes a pressurizing rod, a pressure sensor, and a drive unit; the lower end of the pressurizing rod is provided with a contoured pressure head adapted to the warping profile of the substrate; the pressure sensor detects the pressurizing pressure in real time; and the drive unit is driven by a servo motor, which can realize precise adjustment and pressure holding control according to the instructions of the control system.

[0024] The temperature compensation component includes a heating element disposed inside the conformal pressure head and a temperature sensor capable of acquiring the temperature of the upper and lower surfaces of the substrate in real time. The temperature sensor acquires the temperature of the upper and lower surfaces of the substrate in real time and transmits the data to the control system to realize closed-loop control of the heating power and ensure the temperature uniformity of the substrate.

[0025] The control system includes a microcontroller, a touch screen, and a data storage module. It can input information such as substrate size and material parameters, automatically calculate the pre-pressing target parameters, and simultaneously display data such as pressure, temperature, and warpage in real time, thereby achieving automated control of the process.

[0026] Furthermore, the base is made of high-temperature resistant ceramic material.

[0027] Furthermore, the surface roughness of the indenter head of the contouring indenter... .

[0028] Furthermore, the detection accuracy of the pressure sensor .

[0029] Furthermore, the heating element is a resistance wire heating element.

[0030] The beneficial effects of this invention are:

[0031] 1. Through closed-loop control of "detection-pre-pressing-directional sintering", the warping deformation after single-sided sintering is effectively corrected. The warping of the final product is ≤0.05mm / mm, which is far superior to the existing technology (usually ≥0.1mm / mm), and significantly improves the flatness of the substrate.

[0032] 2. The special fixture's contouring pressure head and closed-loop pressure control enable precise adjustment of the applied pressure and warpage, avoiding the localized damage caused by traditional heavy-load pressure. The substrate qualification rate has increased from 80% in the existing technology to over 92%.

[0033] 3. The temperature compensation component ensures the uniformity of substrate temperature during sintering. Combined with the stress release effect of pre-pressing and shaping, it reduces the residual stress inside the substrate by 40%-60%, effectively reducing the risk of cracking during the later use of the substrate.

[0034] 4. The adjustable positioning components of the fixture are adapted to substrates of different sizes, and the control system realizes automated operation, reduces manual intervention, improves production efficiency, and is suitable for mass production applications. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the structure of a special fixture.

[0036] In the diagram: 1 is the base, 2 is the positioning component, 3 is the pressurizing component, 4 is the temperature compensation component, 5 is the base plate, and 6 is the control system. Detailed Implementation

[0037] The present invention will now be further described with reference to the accompanying drawings.

[0038] A method for preventing warping during the sintering process of double-sided copper-clad ceramic substrates is based on the core concept of "pre-compression shaping - directional sintering - stress relief", and the specific steps are as follows:

[0039] 1. First sintering process

[0040] The first side of the ceramic substrate is conventionally sintered with a copper sheet to obtain a single-sided copper-coated initial sintered substrate. At this time, the initial sintered substrate warps due to the difference in thermal expansion coefficients between copper and ceramic. The warping direction is that the copper side is raised and the ceramic side is recessed.

[0041] 2. Warp parameter detection

[0042] The maximum warpage of the initial sintered substrate was detected using a laser warpage meter. Record the thickness of the ceramic substrate and the location of the warpage center. Copper sheet thickness and their coefficients of thermal expansion , .

[0043] 3. Pre-compression and shaping process

[0044] The pre-sintered substrate is placed in a special fixture, and the target warpage is set according to the detected warpage parameters and material properties. The pressure component of the fixture applies directional pressure to the raised side (copper side) of the initial sintered substrate, causing the substrate to bend in the opposite direction to the target warp and hold the pressure for 5-10 minutes to eliminate some residual stress.

[0045] 4. Second Surface Sintering Process

[0046] The fixture maintains a pre-pressed state on the initially sintered substrate, and the substrate, still under pre-pressed state, is fed into the sintering furnace for copper sheet sintering on the second side of the ceramic substrate. During the sintering process, the temperature compensation component controls the temperature difference between the upper and lower surfaces of the substrate within ±5℃ to ensure uniform release of thermal stress.

[0047] 5. Cooling and demolding process

[0048] After sintering, the furnace is cooled to room temperature, and the substrate is removed, resulting in a flat double-sided copper-clad ceramic substrate with a final warpage. .

[0049] Design of a dedicated fixture to implement the above method.

[0050] like Figure 1 As shown, the fixture includes a base, a positioning assembly, a pressurizing assembly, a temperature compensation assembly, and a control system, with the specific structure as follows:

[0051] Base 1: Made of high-temperature resistant ceramic material, with a positioning groove on the upper surface that matches the shape of the substrate. A shaping pad is laid at the bottom of the positioning groove to prevent heat from being lost too quickly during sintering.

[0052] Positioning component 2: includes 4 adjustable positioning pins arranged around the positioning groove. The ends of the positioning pins are provided with elastic buffer pads. By adjusting the extension length of the positioning pins, accurate positioning of substrates of different sizes can be achieved, avoiding substrate displacement during the calibration process.

[0053] Pressurization component 3: consists of a pressurization rod, a pressure sensor, and a drive unit; the lower end of the pressurization rod is equipped with a contour-following indenter adapted to the warping profile of the substrate (indenter surface roughness). The pressure sensor detects the applied pressure in real time (detection accuracy). The drive unit uses a servo motor and can achieve precise pressure adjustment and pressure holding control according to the instructions of the control system.

[0054] Temperature compensation component 4: includes a heating element set inside the conforming pressure head and a temperature sensor that can collect the temperature of the upper and lower surfaces of the substrate 5 in real time. The heating element uses resistance wire heating, and the temperature sensor collects the temperature of the upper and lower surfaces of the substrate in real time and transmits the data to the control system to realize closed-loop control of heating power and ensure the temperature uniformity of the substrate.

[0055] Control system 6: Includes a microcontroller, a touch screen and a data storage module. It can input information such as substrate size and material parameters, automatically calculate the pre-pressing target parameters, and simultaneously display data such as pressure, temperature and warpage in real time to realize automated control of the process.

[0056] Example parameters:

[0057] Ceramic substrate: alumina ceramic, dimensions 138mm × 190mm, thickness coefficient of thermal expansion ;

[0058] Copper sheet: Copper, thickness coefficient of thermal expansion ;

[0059] First-side sintering process: sintering temperature 1065℃, holding time 30min, cooling rate 5℃ / min.

[0060] Implementation steps:

[0061] 1. After the first side is sintered, a laser warp meter is used to inspect the initially sintered substrate and measure the maximum warp. The warpage center is located at the geometric center of the substrate.

[0062] 2. Place the initial sintered substrate into the positioning groove of the special fixture, and adjust the positioning pins around the perimeter to fix the substrate and align the warping center with the pressure center of the fixture.

[0063] 3. By inputting parameters into the control system, the target preload warpage is automatically calculated. The driving pressure component applies pressure to the copper sheet side, and the pressure value is fed back to the system by the pressure sensor. When the substrate warpage reaches 1.5mm, the pressure is maintained for 8 minutes.

[0064] 4. Maintain the pressurized state and send the fixture and substrate into the sintering furnace. Set the second sintering parameters: temperature 1065℃, holding time 25min. During the sintering process, the temperature compensation component controls the temperature difference between the upper and lower surfaces of the substrate within 3℃.

[0065] 5. Cool the furnace to room temperature, release the pressure, remove the substrate, and measure the final warpage. The residual stress value was 85 MPa (210 MPa for traditional processes).

[0066] Comparative test results

[0067] Using ceramic substrates and copper sheets with the same parameters, double-sided sintering was performed using the method of this invention and the conventional method (without pre-pressing or special fixtures), respectively. The results are shown in the table below:

[0068]

[0069] Through closed-loop control of "detection-pre-compression-directional sintering", the warping deformation after single-sided sintering is effectively corrected. The warping of the final product is ≤0.05mm / mm, which is far superior to the existing technology (usually ≥0.1mm / mm), and significantly improves the flatness of the substrate.

[0070] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preventing warping during the sintering process of a double-sided copper-clad ceramic substrate, characterized in that, The specific steps are as follows: Step 1: First surface sintering process; The first side of the ceramic substrate is conventionally sintered with the copper sheet to obtain a single-sided copper-coated initial sintered substrate. At this time, the initial sintered substrate warps due to the difference in thermal expansion coefficients between copper and ceramic. The warping direction is that the copper sheet side is convex and the ceramic side is concave. Step 2: Warp parameter detection; The maximum warpage of the initial sintered substrate was detected using a laser warpage meter. Record the thickness of the ceramic substrate and the location of the warpage center. Copper sheet thickness and their coefficients of thermal expansion , ; Step 3: Pre-pressing and shaping process; The pre-sintered substrate is placed in the fixture, and the pre-compression target warpage is set according to the detected warpage parameters and material properties. The pressure component of the fixture applies directional pressure to the copper sheet protrusion side of the initial sintered substrate, causing the substrate to bend in the opposite direction to the target warp and hold the pressure to eliminate some residual stress. Step 4: Second-side sintering process; Maintain the pre-pressed state of the fixture on the initial sintered substrate, and send the substrate in the pre-pressed state into the sintering furnace to sinter copper sheets on the second side of the ceramic substrate. During the sintering process, the temperature compensation component controls the temperature difference between the upper and lower surfaces of the substrate within ±5℃, ensuring uniform release of thermal stress. Step 5: Cooling and demolding process; After sintering, the furnace is cooled to room temperature, and the substrate is removed, resulting in a flat double-sided copper-clad ceramic substrate with a final warpage. .

2. The method for preventing warping during the sintering process of a double-sided copper-clad ceramic substrate according to claim 1, characterized in that: In step three, the target warpage of the preload is set. .

3. The method for preventing warping during the sintering process of a double-sided copper-clad ceramic substrate according to claim 1, characterized in that: In step three, the substrate is bent in the opposite direction to the target warp degree and held under pressure for 5-10 minutes.

4. A special fixture for implementing the anti-warping method during the sintering process of double-sided copper-clad ceramic substrates as described in any one of claims 1-3, characterized in that: It includes a base (1), a positioning component (2), a pressurizing component (3), a temperature compensation component (4), and a control system (6); The upper surface of the base (1) is provided with a positioning groove that matches the shape of the substrate, and a shaping pad is laid at the bottom of the positioning groove; The positioning component (2) includes four adjustable positioning pins arranged around the positioning groove. The ends of the positioning pins are provided with elastic buffer pads. By adjusting the extension length of the positioning pins, accurate positioning of substrates of different sizes can be achieved. The pressurizing component (3) includes a pressurizing rod, a pressure sensor, and a driving unit; the lower end of the pressurizing rod is provided with a contoured pressure head that is adapted to the warping profile of the substrate; the pressure sensor detects the pressurizing pressure in real time; and the driving unit is driven by a servo motor, which can realize precise adjustment and pressure holding control according to the instructions of the control system. The temperature compensation component (4) includes a heating element set inside the conformal pressure head and a temperature sensor that can collect the temperature of the upper and lower surfaces of the substrate (5) in real time. The temperature sensor collects the temperature of the upper and lower surfaces of the substrate in real time and transmits the data to the control system (7) to realize closed-loop control of heating power and ensure the uniformity of substrate temperature. The control system (6) includes a microcontroller, a touch screen and a data storage module. It can input substrate size and material parameter information, automatically calculate pre-pressing target parameters, and simultaneously display the pressure, temperature and warpage data in real time to realize automated control of the process.

5. The special fixture according to claim 4, characterized in that: The base (1) is made of high-temperature resistant ceramic material.

6. The special fixture according to claim 4, characterized in that: The surface roughness of the conformal indenter .

7. The special fixture according to claim 4, characterized in that: The pressure sensor has a detection accuracy of ±0.01MPa.

8. The special fixture according to claim 4, characterized in that: The heating element uses resistance wire heating.

Images