A frequency-bias-free composite screen for LTCC filters and its fabrication method

Abstract

This invention discloses a frequency-bias-free composite screen for LTCC filters and its preparation method. The invention relates to the field of screen printing screen preparation technology. The composite screen includes a stainless steel wire mesh, a polyester mesh fabric, and an outer frame. The polyester mesh fabric is disposed around the stainless steel wire mesh, and the stainless steel wire mesh and polyester mesh fabric intersect. The outer frame is disposed around the polyester mesh fabric. Several sets of openings are provided on the polyester mesh fabric, each opening perpendicular to the corresponding edge of the outer frame. The purpose of this invention's opening design is to block non-equidistant screen tension gradient changes in the graphic during printing, eliminate the degree of deformation of the inner layer, and reduce the frequency bias of the LTCC filter to 0-10MHz, achieving a frequency-bias-free effect for the LTCC filter. This invention solves the problem of discrepancies between the printed line diameter and the designed line diameter in existing screens.

Description

Technical Field

[0001] This invention relates to the field of screen printing stencil preparation technology, specifically to an LTCC filter frequency-bias-free composite stencil and its preparation method. Background Technology

[0002] Low Temperature Co-fired Ceramic (LTCC) technology is a process that involves printing interconnected conductors onto unsintered cast ceramic materials. Through multilayer patterned circuit design, complex structures are stacked and pressed together, then sintered at 850-900℃ to form an integrated multilayer ceramic component. Silver (Ag) is used as the conductor for both internal and external electrodes, and three methods—printing, via filling, and bonding—are employed to connect and conduct the internal and external electrodes, ultimately forming a high-density circuit component body that does not interfere with each other in three-dimensional space. LTCC technology can be widely used in various applications such as ceramic filters, ceramic antennas, antenna switch modules, and integrated modules of active and passive devices. It is a process technology that can further miniaturize and increase the density of circuits. Due to the active deployment of 5G communication equipment in recent years, LTCC filter components have been widely used in aerospace, specific electronic equipment, smart networks, and 5G mobile communications. Furthermore, it has obtained certification in the relatively harsh environments of the automotive electronics industry, such as electric vehicles and autonomous driving, and even in low-Earth orbit satellites.

[0003] Screen printing is best suited for the LTCC (Low Temperature Co-fired Ceramic) RF filter industry. The principle of LTCC filters is based on the interaction of internal LC (inductor-capacitor) structures, allowing electromagnetic fields to resonate through interactive coupling, resulting in a resonant frequency. By controlling the LC value or order, the resonant frequency can be adjusted, or multiple frequency points can be designed, thereby achieving passband or suppression filter characteristics across various frequency bands. In contrast, LC in integrated circuits relies on printing various patterns and interconnecting them vias (VIAs). Through-holes and alignment holes are formed on a green ceramic tape using laser drilling. These vias are small holes drilled in the green ceramic sheet to conduct circuitry between different layers. Thick-film printing technology is used to print and dry the conductor paste, filling the vias and conductor patterns, and drying them according to relevant process temperatures and times to create the desired circuit pattern on the green ceramic tape. These are then stacked together and sintered at 900℃. This process enables miniaturization and high density of circuits, making it particularly suitable for high-frequency communication components. Because screen printing has the advantages of low cost and unlimited pattern forming, the conductor ink can be precisely controlled in terms of wire diameter, turning angle and other diverse layers by adjusting exposure and development parameters and using chemical concentration or soaking conditions during the screen printing process. This technology is irreplaceable among the current multi-repetition printing methods on the market.

[0004] In the electrical characteristics of LTCC filters, frequency is an extremely important indicator. The main reason for frequency deviation comes from the difference between the simulated design dimensions and the printed circuit. Since the simulated design wire diameter is fixed, while the printing process involves the interaction of ink, screen, and green ceramic tape, the biggest variable causing frequency deviation is the unequal difference between the designed wire diameter and the printed wire diameter. This results in the actual wire diameter of the product differing from the simulated design wire diameter. During the printing process, the tension on the screen is not evenly distributed after being squeezed by the squeegee. Even with the same wire diameter on the same screen, the wire diameter will be affected by different degrees of tension depending on the distance from the edge of the screen. This leads to uneven distribution of wire diameter during the printing process. The closer the pattern is to the periphery of the screen, the more obvious the effect of tension. Conversely, the closer to the center of the inner pattern, the less affected by tension. These distributions directly affect the high-frequency electrical deviation of the LTCC filter. Summary of the Invention

[0005] To address the aforementioned technical problems, the present invention aims to provide a frequency-bias-free composite screen printing plate for LTCC filters and its preparation method, thereby resolving the issue of discrepancies between the printed line diameter and the designed line diameter in existing screen printing plates.

[0006] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: a composite screen for LTCC filter without frequency offset is provided. The composite screen includes a stainless steel wire mesh, a tertrol mesh, and an outer frame. The tertrol mesh is disposed around the stainless steel wire mesh, and the stainless steel wire mesh and the tertrol mesh intersect. The outer frame is disposed around the tertrol mesh. The tertrol mesh has a number of openings, and the openings are all perpendicular to the frame edge of the corresponding outer frame.

[0007] Based on the above technical solution, the present invention can be further improved as follows:

[0008] Furthermore, both the stainless steel wire mesh and the outer frame are square.

[0009] Furthermore, the Tedoron mesh fabric has four sets of openings.

[0010] Furthermore, the outer frame is made of aluminum.

[0011] Furthermore, the outer frame dimensions are 320×320mm, 360×360mm, 380×380mm, 450×450mm, 750×750mm, or 800×800mm.

[0012] Furthermore, the opening includes a first row of openings and a second row of openings, which are arranged alternately.

[0013] Furthermore, the lengths of the first row of openings and the second row of openings are equal, and both are 450-550μm.

[0014] Furthermore, the widths of the first and second row openings are equal, and both are 250-350μm.

[0015] Furthermore, the distance between the first row of openings and the second row of openings is 1 / 2 to 1 / 3 of the length of the first row of openings.

[0016] Furthermore, the distance between the first row of openings and the second row of openings is 2-10 times the width of the first row of openings.

[0017] Furthermore, the distance between the second row of openings and the outer frame is three times the length of the second row of openings.

[0018] Furthermore, the tension of the composite screen is 25-40 N / cm.

[0019] Furthermore, the mesh count of the composite screen printing plate is 250-500 mesh.

[0020] Furthermore, the composite screen and the scraper are made of polyurethane.

[0021] Furthermore, the scraper hardness is 60-80.

[0022] This invention also provides a method for preparing the above-mentioned frequency-bias-free composite screen for LTCC filters, comprising the following steps:

[0023] (1) The composite wire mesh is formed by combining the tertrol wire mesh and the stainless steel wire mesh. The composite wire mesh is stretched using a wire mesh stretching machine, and the wire frame and the composite wire mesh are bonded and fixed together.

[0024] (2) Apply a 2-5 μm thick degreaser to the stainless steel wire mesh, then apply a 5-20 μm thick photosensitive paste under dark conditions, and dry it at 40-60℃.

[0025] (3) Select the required layer, attach it to the composite screen, expose it for 10-15 minutes, then place the screen frame in water and shake it to clean it, thus obtaining the printed layer appearance.

[0026] (4) Using laser energy to create openings on the tertrol mesh.

[0027] The present invention has the following beneficial effects:

[0028] 1. Screen printing technology utilizes a tiny gap between the screen and the substrate, allowing the screen mesh to repeatedly move and transfer the image onto the substrate using its own tension. Each printing cycle places a certain load on the screen mesh, making its weight resistance a crucial factor in printing quality. Stainless steel mesh boasts a tensile strength of up to 500 MPa, an elasticity range of 40-60%, and exhibits the best abrasion resistance (approximately 0.1%), along with optimal ink control. This allows for the printing of high-precision LTCC (Low-Temperature Crimping) graphics. Therefore, using stainless steel mesh for screen production combines high strength and high abrasion resistance, meeting the high-precision coating requirements of the LTCC process. To improve printing stability and image resolution, a high mesh count is preferred. A higher mesh count results in higher ink density and clearer digitized images. Even higher mesh counts are used for printing with fine wire diameters. The screen printing plate of this invention is suitable for mesh counts between 250 and 500. A higher mesh count results in more mesh and thus higher overall mesh strength. Using a high mesh count screen printing plate meets the high precision requirements of complex LTCC layers and the pursuit of high-stability process performance. The screen printing plate and squeegee are made of polyurethane, which possesses excellent properties such as high wear resistance, solvent resistance, and good resilience. The squeegee hardness is suitable for a range of 60-80 degrees.

[0029] 2. Uneven tension is a factor that traditional printing screens cannot address through other process conditions. To avoid discrepancies between the printed circuitry and the designed circuitry, the screen design architecture of this invention can completely block uneven tension on the wire diameter during the printing process, thereby eliminating the impact of LTCC electrical frequency deviation. This invention utilizes a design with parallel and perpendicular openings at the distance between the inner graphic and the outer frame of the screen. The openings are arranged in two rows, front and back, which strengthens the blocking effect of printing tension on the deformation of the inner graphic. It avoids the uneven gradient change in screen tension caused by the pressure and compression of the screen by the squeegee when printing metallic ink on the green ceramic tape. This uneven tension gradient distribution leads to varying degrees of wire diameter variation, directly affecting the frequency deviation of the LTCC filter.

[0030] 3. The purpose of the opening design in this invention is to prevent non-uniform tension gradient changes in the screen printing pattern caused by the squeegee's downward pressure during the screen printing process, which directly affects the degree of deformation of the inner pattern. Because the tension generated during printing is not unidirectional, the wire diameter at different locations will experience different degrees of tension variation, resulting in varying degrees of wire diameter variation on the green ceramic tape. This invention uses a screen printing design with openings in both parallel and perpendicular directions at the distance between the inner pattern and the screen frame. Through this opening design, external tension is blocked, allowing for uniform and consistent tension at any position of the inner pattern during printing. This enables precise control of wire diameter variation, achieving the LTCC electrical frequency-distortion-free effect.

[0031] 4. The purpose of the opening design of this invention is to block the non-equidistant changes in screen tension gradient during the printing process, eliminate the degree of deformation of the inner layer, reduce the frequency deviation of the LTCC filter to 0-10MHz, and achieve a frequency deviation-free effect for the LTCC filter.

[0032] 5. This invention is applicable to stainless steel wire mesh and composite mesh. The screen tension is most stable when it is between 25-40 N / cm. After 1000 rounds of repeated printing tests, the graphic change rate can be controlled within 0-0.5%, which meets the process requirements of LTCC diverse layers. Attached Figure Description

[0033] Figure 1 For traditional screen design and for inventing screen design;

[0034] Figure 2 This is a schematic diagram of the composite screen printing plate of the present invention.

[0035] The components include: 1. Stainless steel wire mesh; 2. Tetralon mesh fabric; 3. Outer frame; 4. Openings; 5. First row of openings; 6. Second row of openings. Detailed Implementation

[0036] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are for illustrative purposes only and are not intended to limit the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer should be followed. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0037] Please refer to Figure 1-2A frequency-bias-free composite screen for LTCC filters includes a stainless steel wire mesh 1, a polyester mesh 2, and an outer frame 3. The polyester mesh 2 is disposed around the stainless steel wire mesh 1, and the stainless steel wire mesh 1 and the polyester mesh 2 intersect. The outer frame 3 is disposed around the polyester mesh 2. The polyester mesh 2 has several sets of openings 4, each opening 4 perpendicular to the corresponding edge of the outer frame 3. Both the stainless steel wire mesh 1 and the outer frame 3 are square. The openings 4 include a first row of openings 5 ​​and a second row of openings 6, which are staggered. The lengths of the first row of openings 5 ​​and the second row of openings 6 are equal. All are 450-550μm. The widths of the first row of openings 5 ​​and the second row of openings 6 are equal and both are 250-350μm. The staggered distance between the first row of openings 5 ​​and the second row of openings 6 is 1 / 2-1 / 3 of the length of the first row of openings 5. The distance between the first row of openings 5 ​​and the second row of openings 6 is 2-10 times the width of the first row of openings 5. The distance between the second row of openings 6 and the outer frame 3 is 3 times the length of the second row of openings 6. The tension of the composite screen is 25-40N / cm. The mesh count of the composite screen is 250-500 mesh. The material of the composite screen and the squeegee is polyurethane, and the hardness of the squeegee is 60-80 degrees.

[0038] Example 1:

[0039] A frequency-bias-free composite screen for LTCC filters, the fabrication method of which includes the following steps:

[0040] (1) Combine the Tedoron mesh 2 and the stainless steel wire mesh 1 to form a composite wire mesh (mesh count of 300 mesh). Place the wire mesh frame (material is aluminum, 450×450mm) on the wire mesh stretching machine and stretch the composite wire mesh according to the required force (tension of 30N / cm). After the force is reached, stick the wire mesh to the wire mesh frame. Then use a scraper to evenly scrape the part where the wire mesh and the wire mesh frame are attached to increase the bonding force between the wire mesh and the wire mesh frame. Use an oven set to a temperature range of 40℃ to cure the adhesive and then cut it off.

[0041] (2) Apply degreasing agent to the stainless steel wire mesh 1 that has been stretched into a composite wire mesh. The purpose is to form an oily coating with a thickness of about 3μm on the surface of the mesh, which can increase the oil filtration of the mesh and the adhesion of the photosensitive emulsion. Pour the photosensitive emulsion into the funnel-shaped thickness controller. By adjusting the spacing, the thickness of the photosensitive emulsion can be controlled at 15μm. Ensure that the photosensitive emulsion is evenly applied to the surface of the screen and then dried at 50℃. This photosensitive emulsion treatment process must be carried out in a darkroom to ensure that the quality of the photosensitive emulsion is not damaged by light and affects the integrity of the pattern.

[0042] (3) Select the required layer, attach it tightly to the composite screen, and expose it for 12 minutes to ensure that the area to be exposed to ink is fully exposed. Put the exposed screen frame into water for 4 minutes, gently shake and clean it to rinse the photosensitive paste area with the pattern, and the complete printed layer appearance will be revealed.

[0043] (4) Using laser energy, openings 4 are created around the mesh of the present invention. Through a CO2 laser device, CO2 molecules undergo vibrational energy transitions to generate laser light, ablating the surface of the tertrol mesh 2 to form openings 4 (the four sets of openings 4 are all perpendicular to the corresponding outer frame 3; the openings 4 include a first row of openings 5 ​​and a second row of openings 6, which are staggered; the lengths of the first row of openings 5 ​​and the second row of openings 6 are equal and both are 500 μm). All openings are of equal width and 300μm. The first row of openings 5 ​​and the second row of openings 6 are staggered by a distance of 210μm. The distance between the first row of openings 5 ​​and the second row of openings 6 is 1500μm. The closest distance between the second row of openings 6 and the outer frame 3 is 1500μm. The shorter the laser wavelength, the higher the energy density. The laser wavelength is 8μm, which ensures that a shorter wavelength can generate higher laser energy, thereby achieving a more efficient processing effect. The power is 800 watts. The processing speed and the integrity of the openings 4 can be improved by adjusting the power.

[0044] Example 2:

[0045] A frequency-bias-free composite screen for LTCC filters, the fabrication method of which includes the following steps:

[0046] (1) Combine the Tedoron mesh 2 and the stainless steel wire mesh 1 to form a composite wire mesh (mesh count of 250 mesh). Place the wire frame (material is aluminum, 320×320mm) on the wire mesh stretching machine and stretch the composite wire mesh according to the required force (tension of 25N / cm). After the force is reached, stick the wire mesh to the wire frame. Then use a scraper to evenly scrape the part where the wire mesh and the wire frame are attached to increase the bonding force between the wire mesh and the wire frame. Use an oven set to a temperature range of 30℃ to cure the adhesive and then cut it off.

[0047] (2) Apply degreasing agent to the stainless steel wire mesh 1 that has been stretched into a composite wire mesh. The purpose is to form an oily coating with a thickness of about 2μm on the surface of the mesh, which can increase the oil filtration of the mesh and the adhesion of the photosensitive emulsion. Pour the photosensitive emulsion into the funnel-shaped thickness controller. By adjusting the spacing, the thickness of the photosensitive emulsion can be controlled at 5μm. Ensure that the photosensitive emulsion is evenly applied to the surface of the screen and then dried at 40℃. This photosensitive emulsion treatment process must be carried out in a darkroom to ensure that the quality of the photosensitive emulsion is not damaged by light and affects the integrity of the pattern.

[0048] (3) Select the required layer, attach it to the composite screen, and expose it for 10 minutes to ensure that the area to be exposed to ink is fully exposed. Put the exposed screen frame into water for 3 minutes, gently shake and clean it to rinse the photosensitive paste area with the pattern, and the complete printed layer appearance will be revealed.

[0049] (4) The openings 4 of the mesh plate of the present invention are made around the mesh plate using laser energy. Through the CO2 laser equipment, the CO2 molecules generate laser energy through vibrational energy transition, and ablate the surface of the tertrol mesh 2 to form the openings 4 (the four sets of openings 4 are all perpendicular to the frame edge of the corresponding outer frame 3. The openings 4 include the first row of openings 5 ​​and the second row of openings 6. The first row of openings 5 ​​and the second row of openings 6 are staggered. The lengths of the first row of openings 5 ​​and the second row of openings 6 are equal and are both 450μm. The widths of the first row of openings 5 ​​and the second row of openings 6 are equal and are both 250μm. The staggered distance between the first row of openings 5 ​​and the second row of openings 6 is 150μm. The distance between the first row of openings 5 ​​and the second row of openings 6 is 500μm. The closest distance between the second row of openings 6 and the outer frame 3 is 1350μm). The shorter the laser wavelength, the higher the energy density. The laser wavelength is 5μm to ensure that a shorter wavelength can generate higher laser energy, thereby achieving a more efficient processing effect. The power is 10 watts. The processing speed and the integrity of the openings 4 are improved by adjusting the power.

[0050] Example 3:

[0051] A frequency-bias-free composite screen for LTCC filters, the fabrication method of which includes the following steps:

[0052] (1) Combine the Tedoron mesh 2 and the stainless steel wire mesh 1 to form a composite wire mesh (mesh count of 500 mesh). Place the wire mesh frame (material is aluminum, 800×800mm) on the wire mesh stretching machine and stretch the composite wire mesh according to the required force (tension of 40N / cm). After the force is reached, stick the wire mesh to the wire mesh frame. Then use a scraper to evenly scrape the part where the wire mesh and the wire mesh frame are attached to increase the bonding force between the wire mesh and the wire mesh frame. Use an oven set to a temperature range of 50℃ to cure the adhesive and then cut it off.

[0053] (2) Apply degreasing agent to the stainless steel wire mesh 1 that has been stretched into a composite wire mesh. The purpose is to form an oily coating with a thickness of about 5μm on the surface of the mesh, which can increase the oil filtration of the mesh and the adhesion of the photosensitive emulsion. Pour the photosensitive emulsion into the funnel-shaped thickness controller. By adjusting the spacing, the thickness of the photosensitive emulsion can be controlled at 20μm. Ensure that the photosensitive emulsion is evenly applied to the surface of the screen and then dried at 60℃. This photosensitive emulsion treatment process must be carried out in a darkroom to ensure that the quality of the photosensitive emulsion is not damaged by light and affects the integrity of the pattern.

[0054] (3) Select the required layer, attach it to the composite screen, and expose it for 15 minutes to ensure that the area to be exposed to ink is fully exposed. Put the exposed screen frame into water for 5 minutes, gently shake and clean it, and rinse the photosensitive paste area with the pattern clean to reveal the complete printed layer appearance.

[0055] (4) Using laser energy, openings 4 are created around the mesh of the present invention. Through a CO2 laser device, CO2 molecules undergo vibrational energy transitions to generate laser light, ablating the surface of the tertrol mesh 2 to form openings 4 (the four sets of openings 4 are all perpendicular to the corresponding outer frame 3; the openings 4 include a first row of openings 5 ​​and a second row of openings 6, which are staggered; the lengths of the first row of openings 5 ​​and the second row of openings 6 are equal and both are 550 μm). All openings are of equal width and 350μm. The distance between the first row of openings 5 ​​and the second row of openings 6 is 275μm. The distance between the first row of openings 5 ​​and the second row of openings 6 is 3500μm. The closest distance between the second row of openings 6 and the outer frame 3 is 1650μm. The shorter the laser wavelength, the higher the energy density. The laser wavelength is 11μm, which ensures that a shorter wavelength can generate higher laser energy, thereby achieving a more efficient processing effect. The power is 1000 watts. The processing speed and the integrity of the openings 4 can be improved by adjusting the power.

[0056] Comparative Example 1:

[0057] A traditional screen printing plate, the preparation method of which includes the following steps:

[0058] The opening step (4) is not included; the rest is the same as in Example 1.

[0059] Test case

[0060] Printing was performed using the screen printing plate prepared in Example 1 and the conventional screen printing plate in Comparative Example 1, and the results are as follows: (see Table 1)

[0061] 1. With a wire diameter of 150μm, using a traditional screen printing plate results in a wire diameter variation of 1-5%, affecting the frequency offset by 30-60MHz. Using the screen printing plate of this invention, the wire diameter variation is 0-0.5%, affecting the frequency offset by 0-10MHz.

[0062] 2. With a wire diameter of 120μm, using traditional screen printing plates results in a wire diameter variation of 1-5%, affecting the frequency offset by 30-60MHz. Using the screen printing plate of this invention, the wire diameter variation is 0-0.5%, affecting the frequency offset by 0-10MHz.

[0063] 3. With a wire diameter of 100μm, using a traditional screen printing plate results in a wire diameter variation of 1-5%, affecting the frequency offset by 40-80MHz. Using the screen printing plate of this invention, the wire diameter variation is 0-0.5%, affecting the frequency offset by 0-10MHz.

[0064] 4. With a designed wire diameter of 75μm, using traditional screen printing plates results in wire diameter variations of 1-6%, affecting frequency offset by 50-90MHz. Using the screen printing plate of this invention, wire diameter variations are 0-0.5%, affecting frequency offset by 0-10MHz.

[0065] 5. With a wire diameter of 50μm, using a traditional screen printing plate results in a wire diameter variation of 2-8%, affecting the frequency offset by 75-120MHz. Using the screen printing plate of this invention, the wire diameter variation is 0-0.5%, affecting the frequency offset by 0-10MHz.

[0066] Table 1 Comparison of the effects of traditional screen printing and the screen printing of this invention.

[0067]

[0068] As shown in Table 1, using traditional screen printing plates for wire diameters of 50-150μm results in varying degrees of wire diameter variation, ranging from 1% to 8%. This variation can cause frequency offset changes of 30-120MHz, leading to severe electrical deviations and drastic yield losses. However, the screen printing plate of this invention allows for precise control of wire diameter after printing, with variations of only 0-0.5% across different wire diameters—a negligible difference. The impact on frequency offset is only 0-10MHz. Without altering the manufacturing process, a completely frequency-off effect can be achieved, effectively improving product quality.

[0069] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. 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 LTCC filter drift-free composite screen, characterized by, The composite screen includes a stainless steel wire screen (1), a Teflon screen cloth (2) and an outer frame (3), the Teflon screen cloth (2) is arranged at the periphery of the stainless steel wire screen (1), and the stainless steel wire screen (1) and the Teflon screen cloth (2) are connected, the outer frame (3) is arranged at the periphery of the Teflon screen cloth (2), and a plurality of groups of openings (4) are arranged on the Teflon screen cloth (2), and the openings (4) are perpendicular to the frame edges of the corresponding outer frame (3); The preparation method of the LTCC filter non-frequency offset composite screen comprises the following steps: (1) composite the Teflon screen cloth (2) and the stainless steel wire screen (1) to form a composite screen, stretch the composite screen by using a screen stretching machine, and fix the screen frame and the composite screen; (2) apply a 2-5 μm thick degreasing agent on the stainless steel wire screen (1), then apply a 5-20 μm thick photosensitive paste in dark conditions, and dry at 40-60°C; (3) select the required layer, tightly paste the composite screen, expose for 10-15 min, then place the screen frame in water and oscillate to clean, and obtain the printed layer appearance; (4) design the openings (4) on the Teflon screen cloth (2) by using laser energy; The stainless steel wire screen (1) and the outer frame (3) are both square, the openings (4) include a first row of openings (5) and a second row of openings (6), the first row of openings (5) and the second row of openings (6) are arranged in an interlaced and spaced manner, the lengths of the first row of openings (5) and the second row of openings (6) are equal and are 450-550 μm, the widths of the first row of openings (5) and the second row of openings (6) are equal and are 250-350 μm, the interlaced distance of the first row of openings (5) and the second row of openings (6) is 1 / 2-1 / 3 of the length of the first row of openings (5), the spacing distance of the first row of openings (5) and the second row of openings (6) is 2-10 times the width of the first row of openings (5), the distance of the second row of openings (6) from the outer frame (3) is 3 times the length of the second row of openings (6), the tension of the composite screen is 25-40 N / cm, the mesh number of the composite screen is 250-500 meshes, and the material of the scraper matched with the composite screen is polyurethane, and the hardness of the scraper is 60-80 degrees.

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