Within these two primary stages—drawing and drying—there are numerous smaller procedures that contribute to the overall process. Below is a detailed look at the key steps involved:
### 1. Screen Preparation
The screen preparation process involves several critical steps. These are as follows:
**Steps for Screen Preparation:**
– **Cleaning the Screen Frame**
– **Level Checking**
– **Applying the Bottom Layer Glue**
– **Tensioning the Mesh**
– **Measuring the Tension**
– **Applying Glue**
– **Lowering the Mesh**
– **Sealing the Edges**
– **Storage**
**Detailed Process:**
– **Cleaning the Screen Frame:** Since the screen frame is reused multiple times, it often has residual adhesive, mesh remnants, and other debris. These must be thoroughly cleaned to prevent contamination and ensure proper adhesion between the mesh and the frame.
– **Level Checking:** The screen frame should be placed on a flat surface to check for any deformation. If the frame is warped, it needs to be leveled to ensure the mesh tension is evenly distributed during the printing process.
– **Applying Bottom Layer Glue:** Once the screen frame is cleaned and leveled, a layer of Nanbao grease (without hardener) is evenly applied to the mesh. This helps enhance the adhesion between the mesh and the frame during the drawing process.
– **Tensioning the Mesh:** The mesh is then carefully stretched over the frame. It is important to apply uniform tension across the mesh to ensure consistent printing results.
– **Measuring the Tension:** After the mesh is stretched, the tension is measured using a tension meter to ensure it falls within the required range for optimal screen printing.
– **Applying Glue:** Once the mesh is correctly tensioned, another layer of adhesive is applied to seal the mesh securely to the frame, ensuring a durable bond that will withstand repeated use.
– **Sealing the Edges:** The edges of the mesh are sealed to prevent any fraying and ensure the screen is fully prepared for printing.
– **Storage:** Once all steps are completed, the screen is stored in a clean, dry environment to maintain its quality until it is ready for use.
### Conclusion
The screen preparation process is critical in PCB screen printing as it directly affects the quality and precision of the final product. Proper cleaning, leveling, tensioning, and adhesive application ensure that the mesh adheres well to the frame, providing a stable and consistent surface for printing. Following these steps meticulously will contribute to higher efficiency and better performance during PCB manufacturing.
### Comprehensive Guide to PCB Screen Printing and Mesh Preparation
**Introduction:**
This article provides a step-by-step approach to the preparation of PCB screen printing, mesh selection, tensioning, gluing, and other essential steps involved in the process. The aim is to ensure high-quality screen printing with precise mesh handling and accurate silk-screen specifications, which are critical for PCB assembly and functionality.
### **Mesh Preparation and Gluing Process**
1. **Initial Gluing:**
After applying glue for approximately 10 minutes, place the screen frame onto a flat surface, such as a drawing table, to adjust the relative position and height. Carefully select the mesh size, loosen the surrounding grippers, and lay the mesh on the frame. Ensure that the mesh is evenly spread and clamped into the gripper, with no wrinkles. Make sure that the four corners of the mesh are slightly looser than the rest. Tighten the grippers securely to prevent any gaps between the clamp and mesh.
2. **Mesh Tensioning:**
– First tension: Set the tension to 26, and after 5 minutes, adjust it to 24.
– Second tension: Increase the tension to 28, and after another 5 minutes, adjust it to 26.
– Third tension: Set the tension to 32, and after 5 minutes, adjust it to 30.
– Fourth correction: Maintain a tension of 32, and allow it to stabilize for 20 minutes. After this, the final tension should be around 30.
After 72 hours of curing, the tension of the screen will stabilize, allowing for optimal performance during screen printing.
3. **Glue Application:**
Use a small brush to apply an even coat of glue to both the screen frame and the mesh surface. Be cautious not to let any glue drop into the center of the screen. Allow the glue to dry for about 8 minutes. Once dry, use a spatula to evenly spread the glue across the glued surfaces. It is essential to use an air blower for enhanced drying. After the glue fully cures, proceed with mesh installation.
4. **Mesh and Frame Finishing:**
Trim any excess mesh around the screen and record relevant details such as the installation date, mesh type, and tension on the screen frame for future reference. To prevent white water penetration, seal the inner corners of the frame with red glue, followed by a layer of waterproof tape around the mesh. This additional sealing step helps safeguard against any contamination or damage to the mesh during usage.
### **Mesh Cleaning and Preprocessing**
1. **Net Washing:**
– **New mesh**: Use grinder paste to remove any grease or contaminants.
– **Old mesh**: Apply ghost paste to clean the graphics, followed by pulp powder to remove net pulp and blue oil.
– **Final Rinse**: Wash the net with a detergent solution, followed by a high-pressure rinse using a water gun, and a final clean rinse with pure water.
2. **Drying the Mesh:**
The oven temperature for drying the mesh should be set below 48°C to avoid warping or damage. After drying, the mesh is ready for the next steps.
### **Film Application and Mesh Sizing**
1. **Film Selection and Application:**
Once the net is clean, choose a film that is 20% larger than the mesh. To apply the film, use a triangle ruler to press one edge of the film onto the mesh, and then gradually scrape it across the surface to ensure even coverage. Use a squeegee to press down and remove any excess water from the film. Let the film dry completely.
2. **Mesh Sizing:**
After the mesh is dry, apply a mesh pulp. The mesh pulp application process is as follows:
– **Base Layer**: Apply green oil, base oil, and surface oil in layers. Green oil should be applied three times (every 10 minutes), while the base oil and surface oil are applied in smaller quantities.
– **Blue Gum Application**: Tear off the film with a 50K water film, followed by applying 20 mesh pulp, ensuring that it is evenly spread and scraped three times. Once completed, the mesh should be dried.
For best results, a fully automatic sizing machine can be used to streamline the process and ensure uniform application.
### **Mesh Specifications and Printing**
1. **Mesh Selection:**
The following mesh types are typically used in PCB screen printing:
– **120T, 100T, 110T**: For general use, including green oil, base oil, and surface oil.
– **51T**: For carbon oil.
– **24T**: For blue glue, photosensitive line, and thermosetting green oil.
– **77T**: For green oil.
2. **Film Exposure:**
– Apply the required engineering graphic film onto the selected position on the mesh.
– Use an exposure machine with a 3000W spotlight.
– **Standard Exposure Time**: 60-80 seconds for general oil, 80-100 seconds for green oil, and 40-60 seconds for base oil.
– For carbon oil and blue gum, the exposure time should be between 350-400 seconds.
After exposure, rinse the mesh with pressurized water and dry it thoroughly.
3. **Sealing and Finishing:**
Seal the outer edge of the graphic film with blue oil or sealing paste to prevent exposure-related issues. Allow it to dry completely. Ensure all components are checked for accuracy, and any necessary repairs are made. Record the completion date and relevant details for reference.
### **PCB Silk-Screen Specifications**
1. **General Requirements:**
– **Component Identification**: All components, mounting holes, and positioning holes should have corresponding silk-screen marks for easy installation. Use labels such as H1, H2, etc., for mounting holes.
– **Character Orientation**: Silk-screened characters should follow a left-to-right, bottom-to-top pattern. For polarized components (e.g., electrolytic capacitors and diodes), ensure consistent orientation across all units.
– **No Silk Screen on Pads**: Do not apply silk screen to device pads or tin tracks to ensure proper soldering.
– **Clear Polarity Markings**: Mark the polarity direction of polarized components clearly.
2. **Design and Layout Specifications:**
– The silk screen should not obscure the device number after installation, and it must not be applied to via holes or pads to avoid issues during soldering.
– **Silk-Screen Spacing**: Ensure the minimum spacing between silk-screen characters is at least 5 mils.
– **Connector Orientation**: Clearly mark the direction of directional connectors on the silk screen.
3. **Additional Information:**
– The PCB should include information such as the board name, version number, and production date.
– Include a barcode frame (42×6) on the PCB for easy scanning during assembly and testing.
– The PCB must display anti-static symbols and other relevant manufacturer details to ensure safe handling.
### **Conclusion**
This guide outlines the critical steps for effective PCB screen printing, from mesh preparation and gluing to film application and drying. Proper handling of mesh tensioning, film exposure, and silk-screen printing specifications is essential for high-quality PCB production. However, these steps are general guidelines; real-world applications may require flexibility and adjustment based on specific project needs. It is essential to continuously learn, adapt, and improve processes to achieve the best results.
