Method for improving grain stability of rolled oxygen-free copper strip at high temperature

Abstract

The invention discloses a method for improving grain stability of a calendered oxygen-free copper strip at high temperature, which comprises the following steps of: sequentially carrying out rough rolling, primary annealing, finish rolling, secondary annealing and finish rolling on an oxygen-free copper blank to obtain the calendered oxygen-free copper strip, wherein the oxygen content of the oxygen-free copper blank ranges from 3 ppm to 10 ppm; the tension applied in the secondary annealing process is 1-5 MPa; and the finish rolling procedure is multi-pass cold rolling, the single-pass machining rate is 1-15%, and the total machining rate is 5-20%. The grain stability of the prepared rolled oxygen-free copper strip at high temperature is cooperatively controlled through the parameters, so that grains of the rolled oxygen-free copper strip grow up uniformly in the high-temperature sintering process, and the grain size is small and uniform. According to the prepared oxygen-free copper strip, the tensile strength of the strip is improved while high conductivity is guaranteed, the use requirement for the oxygen-free copper strip when a high-temperature sintering process is adopted in electronic packaging is met, and application of an IGBT ceramic copper-clad plate and the like to electronic packaging devices is achieved.

Description

Technical Field

[0001] This invention relates to the field of copper alloy material processing technology, specifically to a method for improving the grain stability of rolled oxygen-free copper strip at high temperatures. Background Technology

[0002] Oxygen-free copper has an oxygen content of less than 0.003% and an impurity content of less than 0.05%. It possesses excellent electrical and thermal conductivity, weldability, low-temperature performance, corrosion resistance, and mechanical properties, making it widely used in electronic components such as integrated circuits, semiconductor devices, and miniature transformers. With the development of electronic packaging technology, oxygen-free copper strip has been applied to electronic packaging devices such as ceramic copper-clad laminates.

[0003] Common methods for preparing ceramic-clad copper (DCB) are direct bonding copper plating and active metal brazing (AMB). Direct bonding copper plating involves bonding a 0.15–0.3 mm thick copper foil to a 0.25 mm or 0.38 mm thick Al2O3 ceramic substrate on one or both sides at high temperature (≥1000 °C) in a flowing atmosphere (N2+O2) to produce the DCB composite material. Active metal brazing, on the other hand, is a brazing method that fuses metal and ceramic without metallization. It uses a high-temperature vacuum brazing process to braze pure copper onto a ceramic substrate, producing the DCB composite material. Its effective temperature range is 700–950 °C, and due to the difference in thermal expansion properties between ceramic and copper, the actual high-temperature holding time is longer.

[0004] Therefore, the key to the preparation of ceramic copper-clad laminates lies in high-temperature sintering. However, during the high-temperature sintering process, the grains of oxygen-free copper raw materials are prone to coarsening and growing and are unevenly distributed, which leads to poor subsequent etching accuracy and seriously affects product quality and reliability.

[0005] Current methods for improving the high-temperature stability of rolled copper strip grains mainly include ppm-level trace composition control and processing microstructure control techniques. However, achieving ppm-level composition homogenization in actual production is difficult and can easily lead to abnormal grain growth at high temperatures, which is detrimental to the mass production of oxygen-free copper strips. In processing microstructure control, grain boundary engineering can regulate the microstructure of rolled copper strips to achieve grain size stability at 900 ℃ for 60 min. However, since this is mainly controlled through annealing twinning, grain boundary coarsening becomes severe at higher temperatures (e.g., 1000 ℃) and with longer actual sintering holding times, which is detrimental to the subsequent etching precision control. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a method for improving the grain stability of rolled oxygen-free copper strip at high temperatures. By controlling the oxygen content of the oxygen-free copper billet, the tension of 1-5 MPa in the second annealing process, and the final rolling processing rate, the grain stability of the rolled oxygen-free copper strip at high temperatures is controlled in a coordinated manner, resulting in uniform grain growth of the rolled oxygen-free copper strip during high-temperature sintering, with smaller and more uniform grain size.

[0007] A method for improving the grain stability of rolled oxygen-free copper strip at high temperatures includes the following steps: Oxygen-free copper billets are subjected to rough rolling, first annealing, fine rolling, second annealing and final rolling processes in sequence to obtain rolled oxygen-free copper strip. The oxygen content of the oxygen-free copper billet is 3-10 ppm; the tension applied in the second annealing process is 1-5 MPa. The final rolling process is a multi-pass cold rolling process, with a single-pass processing rate of 3-20% and a total processing rate of 7-28%.

[0008] In this invention, by controlling the oxygen content of the oxygen-free copper billet, the tension of 1-5 MPa in the second annealing process, and the processing rate of the final rolling, the rolled oxygen-free copper strip is synergistically controlled to achieve high-temperature grain size stability while ensuring high conductivity (>98% IACS) and a certain strength (tensile strength ≥350 MPa). Given that copper alloys have a melting point of 1086 ℃, and grains will grow at temperatures up to 1000 ℃, this invention ensures that the grain growth process is controllable and uniform, resulting in a small average grain size (≤100 μm) and relatively uniform size distribution after high-temperature sintering.

[0009] Oxygen-free copper with an oxygen content of 3-10 ppm is used to manufacture high-temperature sintering and thermally stable rolled oxygen-free copper strip. This ensures the high electrical and thermal conductivity of the oxygen-free copper strip and the pinning effect of oxygen on grain boundaries during high-temperature sintering, while avoiding the impact of uneven oxygen content distribution on grain size uniformity during high-temperature sintering and preventing the formation of excessively large grains. Applying a tension of 1-5 MPa during the second annealing can suppress the recrystallization growth rate and control the preferential direction of recrystallized grain growth, thereby further refining the grain size of the finished rolled oxygen-free copper strip and improving the uniformity of grain size distribution. After the second annealing, it undergoes multi-pass cold rolling with a total processing rate of 7%-28%. This ensures the dislocation density of the oxygen-free copper strip while controlling the texture composition, achieving a copper texture area ratio of 25%-35%, thereby preventing abnormal grain growth at high temperatures, avoiding grain boundary stress concentration, and reducing the tendency for grain boundary coarsening at high temperatures.

[0010] The method for preparing rolled oxygen-free copper strip provided by this invention improves the grain size stability of oxygen-free copper strip at high temperature while ensuring the uniformity of grain distribution. The resulting oxygen-free copper strip improves the tensile strength of the strip while ensuring high conductivity, meeting the requirements for the use of oxygen-free copper strip in electronic packaging using high-temperature sintering processes, and enabling the application of IGBT ceramic copper-clad laminates and other electronic packaging devices.

[0011] Preferably, the roughing process is a multi-pass cold rolling process, wherein the single-pass processing rate is 20%~60%, and the total processing rate is 75%~90%.

[0012] In this invention, the rough rolling process provides deformation energy for recrystallization in the first annealing process, and works in conjunction with the first annealing process to obtain a smaller recrystallized grain structure, which helps to make the final product structure uniform and fine.

[0013] More preferably, the thickness of the oxygen-free copper strip obtained by the rough rolling process is 1.5~3.5 mm.

[0014] Preferably, the first annealing process is a two-stage annealing process, wherein the annealing process is as follows: first, the temperature is held at 250~320℃ for 1~3 hours, and then heated to 360~440℃ and held for 2~5 hours.

[0015] In this invention, the deformed grains can be restored in the first stage by means of a two-stage process, and the second stage is uniform recrystallization. After annealing, a fully recrystallized grain structure is obtained. Since the deformed grains will grow while recrystallizing, the holding time needs to be shortened in order to control the grain size. Therefore, the annealing conditions need to be controlled within the above range.

[0016] More preferably, the grains in the oxygen-free copper strip obtained after the first annealing process are recrystallized grains, wherein the average grain size is ≤28 μm.

[0017] Preferably, the finishing rolling process is a multi-pass cold rolling process, wherein the single-pass processing rate is 7% to 35%, and the total processing rate is 40% to 70%.

[0018] In this invention, the finishing rolling process employs multi-pass deformation, which mainly aims to achieve a high amount of deformation while ensuring the uniformity of the deformed structure. However, since the second annealing requires recrystallization of the grains without grain growth, the total deformation of the finishing rolling process needs to be controlled.

[0019] More preferably, the thickness of the oxygen-free copper strip obtained after the finishing rolling process is 0.5~0.8 mm.

[0020] Preferably, in the second annealing process, the annealing temperature is 360~440 ℃ and the time is 1~6 h.

[0021] More preferably, the grains in the oxygen-free copper strip obtained after the second annealing process are recrystallized grains, wherein the average grain size is 3~10 μm.

[0022] Preferably, the thickness of the oxygen-free copper strip obtained after the final rolling process is 0.15~0.4 mm.

[0023] The present invention also provides a rolled oxygen-free copper strip obtained by the above method.

[0024] Preferably, in the rolled oxygen-free copper strip, the area ratio of deformed grains in the internal grain structure is ≥70%, the area ratio of copper texture is 25%~35%, and the average grain size is 2~6 μm.

[0025] In this invention, the internal grain structure can be divided into three types of grains: deformed, substructured, and recrystallized, determined by electron backscatter diffraction (EBSD) test according to orientation difference (KAM); copper texture is the name of the texture, which is a type of deformed texture and is more common in copper alloys.

[0026] Preferably, the rolled oxygen-free copper strip has a conductivity >98% IACS and a tensile strength ≥350 MPa.

[0027] Preferably, the average grain size of the rolled oxygen-free copper strip after sintering at 1000 °C is 65~100 μm.

[0028] The present invention also provides the application of the above-mentioned rolled oxygen-free copper strip in the preparation of ceramic copper-clad laminates.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows: The method for preparing rolled oxygen-free copper strip provided by this invention improves the grain size stability of oxygen-free copper strip at high temperature while ensuring the uniformity of grain distribution. The resulting oxygen-free copper strip improves the tensile strength of the strip while ensuring high conductivity, meeting the requirements for the use of oxygen-free copper strip in electronic packaging using high-temperature sintering processes, and enabling the application of IGBT ceramic copper-clad laminates and other electronic packaging devices. Attached Figure Description

[0030] Figure 1 The image shows the microstructure of the rolled oxygen-free copper strip prepared in Example 1, with the illustration in the lower right corner.

[0031] Figure 2 Metallographic image of the rolled oxygen-free copper strip prepared in Example 1 after being held at 1000℃ for 30 min. Detailed Implementation

[0032] The present invention will be further described in detail below with reference to the embodiments, but the implementation of the present invention is not limited to the following embodiments.

[0033] All raw materials used in this invention are commercially available.

[0034] Example 1 (1) Rough rolling: The oxygen-free copper billet with an oxygen content of 3.5 ppm is processed in 4 times to reduce the size from 12 mm → 6.5 mm → 3.5 mm → 2 mm → 1.3 mm, with a total processing rate of 89%; (2) First annealing: The oxygen-free copper strip after rough rolling is placed in a bell furnace for annealing. First, it is heated to 280 ℃ and held for 2 h, and then heated to 400 ℃ and held for 4 h. (3) Finish rolling: The oxygen-free copper strip after the first annealing is processed in 4 times to reduce the size from 1.3 mm to 0.9 mm to 0.7 mm to 0.55 mm to 0.45 mm, with a total processing rate of 65%; (4) Second annealing: The oxygen-free copper strip obtained by fine rolling is subjected to tension annealing using a bell furnace device. First, a tension of 2.5 MPa is applied through a winding device, and the tension is fixed at both ends of the strip to ensure that it remains constant during the annealing process. The strip is held at 440 ℃ for 4 h to obtain the oxygen-free copper strip after the second annealing. (5) Final rolling: The oxygen-free copper strip after the second annealing is processed twice to reduce the size from 0.45 mm to 0.4 mm to 0.35 mm, with a total processing rate of 22%, to obtain rolled oxygen-free copper strip.

[0035] Examples 2-9 and Comparative Examples 1-7 The preparation method is the same as in Example 1, with the differences shown in the table below.

[0036] Table 1: Differences between the preparation methods of Examples 1-9 and Comparative Examples 1-7 The average grain size, internal grain structure deformation grain area ratio, and copper texture area ratio of the rolled oxygen-free copper strips prepared in the examples and comparative examples were tested, and the results are shown in Table 2. Subsequently, the tensile strength, electrical conductivity, and Vickers hardness of the rolled oxygen-free copper strips were tested, and the results are shown in Table 3. The test methods are as follows: Average grain size test: The metallographic structure of the material was observed using a fluorescence metallographic microscope, and then the grain area was counted using Image Pro Plus software. The equivalent circle diameter calculated from the area was the grain diameter. Internal grain structure deformation, grain area ratio and copper texture area ratio: The rolled surface structure was tested using a thermal field emission scanning electron microscope equipped with an EBSD probe, and the ratio data were obtained by Aztec Crystal processing and analysis.

[0037] Tensile strength test: The tensile test was conducted using an electronic universal tensile testing machine at a tensile rate of 1 mm / min. The stress-strain curves of the oxygen-free copper in the rolling direction (RD) were measured. Conductivity test: The conductivity of oxygen-free copper was tested using an eddy current conductivity meter, and the average of six results was taken. Vickers hardness test: The hardness of oxygen-free copper at room temperature was determined using a Vickers hardness tester. The load used in the test was 0.2 kg, and the load holding time was 15 s. The average value of eight results was taken. Table 2: Microstructure of rolled oxygen-free copper strips from Examples 1-9 and Comparative Examples 1-7 Table 3: Performance Tests of Rolled Oxygen-Free Copper Strips from Examples 1-9 and Comparative Examples 1-7 Note: - indicates that no test was conducted.

[0038] As shown in Tables 2 and 3, the average grain size of the rolled oxygen-free copper strips obtained in the examples is 3~4 μm (e.g., Figure 1 The IPF of Example 1 is similar to that of the comparative example, slightly smaller, but the proportion of deformed grains and copper texture measured by EBSD is significantly higher than that of the comparative example. The average grain size of the sample in Example 1, measured after being heated at 1000 °C for 30 min, is less than 100 μm, both smaller than that of the comparative example. Figure 2 In Example 1, the grain size was relatively uniform, with no abnormally large grains. An average grain size of less than 100 μm is a key parameter requirement for copper strip products in ceramic copper cladding processes to ensure the copper cladding effect.

[0039] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for improving the grain stability of rolled oxygen-free copper strip at high temperatures, characterized in that, Includes the following steps: Oxygen-free copper billets are subjected to rough rolling, first annealing, fine rolling, second annealing and final rolling processes in sequence to obtain rolled oxygen-free copper strip. The oxygen content of the oxygen-free copper billet is 3-10 ppm; the tension applied in the second annealing process is 1-5 MPa. The final rolling process is a multi-pass cold rolling process, with a single-pass processing rate of 3-20% and a total processing rate of 7-28%.

2. The method for improving the grain stability of rolled oxygen-free copper strip at high temperature according to claim 1, characterized in that, The roughing process is a multi-pass cold rolling process, wherein the single-pass processing rate is 20%~60%, and the total processing rate is 75%~90%.

3. The method for improving the grain stability of rolled oxygen-free copper strip at high temperature according to claim 1, characterized in that, The first annealing process is a two-stage annealing process, which involves first holding at 250~320 ℃ for 1~3 h, and then heating to 360~440 ℃ and holding for 2~5 h.

4. The method for improving the grain stability of rolled oxygen-free copper strip at high temperature according to claim 1, characterized in that, The finishing rolling process is a multi-pass cold rolling process, in which the single-pass processing rate is 7%~35% and the total processing rate is 40%~70%.

5. The method for improving the grain stability of rolled oxygen-free copper strip at high temperature according to claim 1, characterized in that, In the second annealing process, the annealing temperature is 360~440 ℃ and the time is 1~6 h.

6. The rolled oxygen-free copper strip obtained by the method according to any one of claims 1 to 5.

7. The rolled oxygen-free copper strip according to claim 6, characterized in that, In the aforementioned rolled oxygen-free copper strip, the area ratio of deformed grains in the internal grain structure is ≥70%, the area ratio of copper texture is 25%~35%, and the average grain size is 2~6 μm.

8. The rolled oxygen-free copper strip according to claim 6, characterized in that, The aforementioned rolled oxygen-free copper strip has a conductivity >99% IACS and a tensile strength ≥320 MPa.

9. The rolled oxygen-free copper strip according to claim 6, characterized in that, When the rolled oxygen-free copper strip is sintered at 1000℃, the average grain size is 65~100 μm.

10. The application of the rolled oxygen-free copper strip according to any one of claims 6 to 9 in the preparation of ceramic copper-clad laminates.

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