I. Introduction
The screen printing process has been widely used in the production process of printed boards. Not only the transfer of circuit patterns of single-sided boards, double-sided boards, but also multi-layer printed boards, screen printing processes have been adopted for solder resists, character marks, etc. . With the development of economic technology, it is increasingly required to improve product accuracy and pass rate, ensure product quality, and reduce costs, thereby achieving better economic benefits. In recent years, fine lines have also been manufactured using screen printing processes, and high position accuracy is required. This article analyzes several factors that affect the accuracy of screen printing on the printed board during the screen printing process based on our experience in screen printing, and puts forward some views on how to improve the accuracy for common discussion.
Second, the factors that affect the accuracy of screen printing
The printing accuracy of the printed board graphics includes the following two aspects: one refers to the small distortion of the printed graphic line width, and the line width accuracy is ± 0.05mm; the second refers to the high precision of the printed graphic position, and the graphic length is within 500mm , The position deviation is less than ± 0.15mm, there are many factors that affect the accuracy of the above two aspects, from the screen plate to the graphic printing production process, there are mainly the following factors:
1. Selection of screen-making materials and printing materials: including screen frame material, screen material, mesh, color and printing material performance;
2. Screen printing equipment, tooling: screen printing machine, positioning tooling, etc .;
3. Production process: including stretch net, screen plate making, printing process, etc .;
4. Working environment conditions: the temperature, humidity and cleanliness of the work site, especially the cleanliness is an important factor affecting the quality of the product;
5. People are the most important factor affecting the quality of the project. The operator must have a high sense of responsibility and a high technical quality.
This article focuses on discussing the principles of the influence of the previous factors on the printing accuracy of printed graphics and the measures and methods to improve accuracy.
3. Improving the quality of screen templates is a prerequisite for ensuring the accuracy of screen printing
The factors that affect the quality of the screen template include the drawing quality of the photographic manuscript, the dimensional stability of the photograph, the copy and the photographic film. The production of silk screen templates involves several factors such as screen frames, silk screens and stretch nets, plate-making and photosensitive materials.
1. The selection of the frame material is generally selected from LY12, LF2 and other hard alloy aluminum square tube profiles, the size is selected to be 20mm wide and 20mm thick, and the four corners are processed by argon arc welding or riveting. The metal frame has high precision and stable dimensions.
2. The choice of wire mesh materials The nylon mesh and polyester mesh are currently the most used. Nylon screen has poor heat resistance, and it is easy to produce thermoplastic deformation after heating, which makes the tension uneven and affects the quality of screen printing. It is not suitable for high-temperature baking for the production of bonded stretch net or silk screen template. Polyester wire mesh has good heat resistance, stable size, and the pattern does not change greatly due to changes in temperature and humidity. In order to ensure the accuracy of screen printing graphics, it is best to use monofilament polyester screen. At the same time, a wire mesh with a high mesh mesh number, a fine wire diameter and a small mesh should be selected. The color of the wire mesh is preferably yellow or brown to prevent vignetting.
3. Stretching the net Stretching the net is an indispensable link. To produce high-precision printed graphics, the stretched net must first meet the quality requirements. It is best to use a pneumatic stretcher.
â‘ The tension of the stretched net is appropriate and even
It is not that the greater the tension, the better. If the tension is too large and exceeds the elastic limit of the material, the wire mesh will lose its elasticity, become brittle and even break. The tension is insufficient and the wire mesh is soft and lacks resilience. It should be based on the rated tension, and then be considered according to the operating conditions to make the tension on the entire screen surface uniform.
â‘¡The warp and weft meshes remain vertical
The warp and weft of the stretched wire mesh should be as vertical as possible to the edge of the mesh frame. When stretching the mesh, the wire mesh is drawn positively, that is, the force is consistent with the wire direction; mobile.
â‘¢ Prevent slack
In order to reduce the stretched net over time, the screen will become looser or looser as you use it, resulting in a decrease in tension. Continuous drawing and repeated tensioning of the net should be used to relax part of the tension before the net is fixed.
4. The screen stencil making process currently uses the most direct method to make a screen stencil. In this manufacturing process, pay attention to the thickness of the sizing coating, and strictly control the drying, exposure, development and other links to obtain high quality. Screen template.
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4. Control various influencing factors in the screen printing process to improve screen printing accuracy
In the process of screen printing graphics processing, first of all, it is required that the printing material has good performance, and secondly, the process factors such as printing equipment, tooling, scraper, screen distance, and printing pressure must be strictly controlled.
1. The printing material must have the following properties:
Good chemical stability;
Suitable drying speed;
Good adhesion to copper-clad plates;
Appropriate viscosity, good thixotropy, suitable color;
Good printing power;
Easy to dry.
2. Equipment tooling
The use of a screen printing machine with higher printing accuracy can eliminate many human instability factors. The positioning tooling can be directly positioned using the machine table longitudinally adjustable positioning pins (the positioning holes can be drilled by CNC drilling).
3. Other process factors
Scraper material, squeegee width, hardness, squeegee angle, cutting edge status and screen pitch, squeegee pressure, etc. are all important, and polyester rubber should be used for squeegee. The width of the scraper is appropriate. If the width is too wide, the graphics will be distorted. The hardness of the scraper is mostly within the range of Shore A60 to 80. The scraping angle is 20 to 30 °. The edge of the blade must be kept flat and sharp. The screen distance is generally 2 ~ 4mm and the screen is parallel to the table. The scraping pressure is suitable for printing clear graphics.
In summary, among the several process factors mentioned above, the main factors that affect the accuracy of the line pattern include printing material, scraper hardness, cutting edge status, scraping pressure and angle, etc., and screen printing equipment, tooling, screen spacing and scraper width The main factors such as the position accuracy of the printed graphics, high-quality screen templates are essential to improve the printing accuracy.
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