Screen printing process has been widely used in the production process of printed circuit boards, not only the transfer of circuit patterns of single-sided, double-sided and multilayer printed boards, but also screen-printing processes such as solder resists and character marks. . With the development of economy and technology, it is increasingly required to improve product accuracy and qualification rate, ensure product quality and reduce costs, so as to obtain better economic benefits. In recent years, fine lines have also been produced using screen printing processes and require high positional accuracy. Based on my experience in screen printing, this article analyzes several factors affecting the accuracy of screen printing in the screen printing process, and puts forward some ideas on how to improve the accuracy for joint discussion.

Second, the factors affecting the screen printing accuracy

Printed board graphic printing accuracy includes the following two aspects: First, refers to the small distortion of the printed graphics line width, line width accuracy of ± 0.05mm; Second, refers to the high precision of the position of printing graphics, graphics length of 500mm The position deviation is less than ±0.15mm. There are many factors affecting the accuracy of the above two aspects. From the screen making process to the graphic printing production process, there are mainly the following factors:

1. The selection of silk screen plate materials and printing materials: including frame material, screen material, mesh number, color and printing material performance;

2. Screen printing equipment, tooling: screen printing machine, positioning fixtures, etc.;

3. Production process: including stretch net, screen plate, printing process, etc.;

4. Working environment conditions: The temperature, humidity and cleanliness of the work site, especially the cleanliness, are important factors affecting product quality;

5. People are the most important factor affecting the quality of the project. The operator must have a high degree of responsibility and high technical quality.

The principle of the influence of the factors of the previous aspects on the precision of printing graphics and the measures and methods for improving the accuracy are discussed here.

Third, improve the quality of the screen template is to ensure the screen printing accuracy of the premise

Factors affecting the quality of the screen stencil include the quality of the photographic manuscript, the photographic quality, the piracy, and the dimensional stability of the photographic negative. The production of screen templates involves a number of factors such as screen frames, screens and stretch nets, plate making and photosensitive materials.

1. The choice of frame material is generally selected LY12, LF2 and other hard alloy aluminum square tube, size selection width 20mm, thickness 20mm, four corners by argon arc welding or riveting processing. The metal frame has high precision and dimensional stability.

2. The choice of screen material The most commonly used is the nylon mesh and polyester mesh. Nylon mesh is poor in heat resistance, easily deformed by heat after being heated, making the tension non-uniform, affecting the quality of screen printing. Bonding stretch net or screen stencil production should not be high-temperature baking. Polyester mesh has good heat resistance, dimensional stability, 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, screens with a higher number of meshes, smaller wire diameters and smaller meshes should be selected. The color of the screens should be yellow or brown to prevent vignetting.

3. Stretch net stretch net is a link that can't be ignored. To produce high-precision print graphics, the stretch net is required to meet the quality requirements first, and it is best to use a pneumatic stretch net machine. The quality requirements of the stretch net are as follows.

1 stretch tension is suitable and uniform

It is not that the greater the tension is, the better the tension is. If the tension exceeds the elastic limit of the material, the screen will lose its elasticity, become brittle and even break. The tension is not enough for the screen to be soft and lacks resilience. Should be in accordance with the rated tension, and then give consideration to the operating conditions, so that the entire screen surface tension even.

2 longitude and latitude of wire kept vertical

The longitude and latitude lines of the stretched mesh should be kept as perpendicular as possible to the edge of the mesh frame. When stretching the mesh, it is pulling the mesh, that is, the force is consistent with the direction of the wire. Second, the mesh held by the mesh clamp can stretch laterally when stretched. mobile.

3 prevent relaxation

In order to reduce the tension of the net with time, the screen will become loose or more loose, resulting in a drop in tension, should continue to pull the net and repeatedly stretch the stretching method, so that part of the tension before loosening in the fixed network to complete.

4. Screen stencil plate making process is currently the most used is the direct method of making screen stencil, in this production process, pay attention to the sizing coating thickness is appropriate, strictly control the drying, exposure, development and other aspects, in order to get high quality Screen stencil.

Fourth, control the various factors in the screen printing process, improve screen printing accuracy

In the process of screen printing graphics processing, the first requirement is that the printing material be of good performance, followed by strict control of printing equipment, tooling, scraper, net distance, squeegee pressure and other technological factors.

1. Ink must have the following properties:

Good chemical stability;
Drying speed is suitable;
Good adhesion to copper-clad plates;
Good viscosity, good thixotropy, suitable color;
Good anti-printing force
Easy to dry.

Equipment equipment

The screen printing machine with high printing precision can eliminate many artificial instability factors. The positioning tooling can directly use the machine table surface to adjust the positioning pin positioning (positioning hole can be controlled by CNC drilling and drilling).

3. Other process factors

Scraper material, scraper width, hardness, squeegee angle, edge state and net distance, squeegee pressure, etc. are all very important, should choose polyester rubber scraper. Scraper width is appropriate, too wide and easy to cause distortion of the graphics, scraper hardness in the majority of Shore A60 ~ 80 range, the squeegee angle of 20 ~ 30 °, the edge should always be kept flat sharp. The net distance is generally 2~4mm and the screen is parallel to the table. It is advisable to print the pressure to print a clear image.

In summary, the main factors influencing the linear accuracy of the line graphics include the printing materials, the hardness of the squeegee, the state of the cutting edge, the squeegee pressure and the angle, and the screen printing equipment, tooling, mesh distance, and the width of the squeegee. Major factors such as the positional accuracy of printed graphics are affected. High-quality screen templates are essential to improve printing accuracy.

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