Discussion on the Process of Improving the Printing Precision of Printed Board

Discussion on the Process of Improving the Printing Precision of Printed Board

I. Introduction

The screen printing process has been widely used in the production process of printed boards. It is not only the transfer of circuit patterns of single-panel, double-panel and multi-layer printed boards, but also the solder resist and character marks. . With the development of economy and technology, it is increasingly required to improve product accuracy and pass rate, ensure product quality, reduce costs, and thus achieve better economic benefits. In recent years, fine lines have also been produced by screen printing processes and require high positional accuracy. Based on the experience in the screen printing work that I carried out, this paper analyzes several factors that affect the precision of screen printing on screen printing, and puts forward some ideas on how to improve the accuracy for joint discussion.

Second, the factors affecting the accuracy of screen printing

Printed board graphic printing accuracy includes the following two aspects: First, the printed graphic line has a small amplitude distortion, and the line width is ±0.05mm; the second is that the printed graphic position accuracy is high, and the graphic length is within 500mm. The positional deviation <±0.15mm affects the above two aspects of precision. From the screen printing to the graphic printing production process, there are mainly the following factors:

1. Screen plate material and printing material selection: including frame material, wire mesh material, mesh number, color and printing 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, cleanliness, especially cleanliness of the workplace are important factors affecting product quality;

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

The focus here is on the principles of the influence of the factors in the first few aspects on the precision of printed graphics and the measures and methods to improve the accuracy.

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

Factors affecting the quality of the screen stencil include the quality of the photographic manuscript, the photographic, reticle, and dimensional stability of the photographic film. The production of screen stencils involves several factors such as frame, screen and stretcher, plate making and photographic materials.

1. The choice of frame material is generally selected from LY12, LF2 and other hard alloy aluminum square tube profiles. The size is 20mm wide and 20mm thick. The four corners are made by argon arc welding or riveting. The metal frame is highly accurate and dimensionally stable.

2. The choice of wire mesh materials is currently used in nylon mesh and polyester mesh. Nylon mesh has poor heat resistance, and it is easy to produce thermoplastic deformation after being heated, which makes the tension uneven and affects the quality of screen printing. Bonding stretch nets or screen stencils are not suitable for high temperature baking. Polyester mesh has good heat resistance and 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 mesh. At the same time, the screen with higher mesh size, thinner wire diameter and smaller mesh should be used. The color of the mesh is yellow or brown, so as to prevent vignetting.

3. Stretching mesh is a must-have link. To produce high-precision printed graphics, firstly, the stretch net is required to meet the quality requirements. It is best to use a pneumatic stretcher. The quality requirements of the stretcher are as follows.

1 stretched net tension is appropriate and uniform

It is not the better the tension, the tension is too large, beyond the elastic limit of the material, the screen will lose its elasticity, become brittle or even break. The tension is insufficient and the screen is soft and lacks resilience. It should be considered according to the rated tension and then according to the working conditions, so that the tension on the entire screen surface is uniform.

2 warp and weft wire remains vertical

The warp and weft of the stretched screen should be as perpendicular as possible to the side of the frame. When the net is stretched, the net is stretched, that is, the force is consistent with the direction of the wire; the second is that the net held by the net can stretch horizontally. mobile.

3 to prevent slack

In order to reduce the stretched mesh over time, the screen will become loose or looser, and the tension will decrease. The method of continuous pulling and repeated tensioning of the stretcher should be adopted to make a part of the tension loose before the fixed net.

4. The most widely used screen stencil making process is the direct method to make the screen stencil. In this production process, pay attention to the appropriate thickness of the sizing coating, strictly control the drying, exposure, development and other aspects to obtain high quality. Screen stencil.

Fourth, control the influencing factors in the screen printing process, improve the accuracy of screen printing

In the process of screen printing graphics processing, the printing performance is first required to be good, and secondly, the printing equipment, tooling, scraper, mesh 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 laminates;
Appropriate viscosity, good thixotropy, suitable color;
Good printing power;
Easy to dry.

2. Equipment tooling

The screen Printing Machine with high printing precision can eliminate many artificial instability factors, and the positioning tool can directly use the longitudinally adjustable positioning pin of the machine table to locate (the positioning hole can be made by CNC drilling).

3. Other process factors

Scraper material, squeegee width, hardness, squeegee angle, edge state and mesh distance, squeegee pressure are all important. Polyester rubber should be used to make squeegee. The width of the squeegee is appropriate, and the pattern is distorted due to excessive width. The hardness of the squeegee is mostly within the range of 60 to 80 Shore A, and the squeegee angle is 20 to 30°. The edge must be kept flat. The mesh distance is generally 2 to 4 mm and the screen is parallel to the table surface, and the printing pressure is preferably printed with a clear pattern.

In summary, among the several process factors, the main factors affecting the accuracy of the line pattern include printing, scraper hardness, edge state, squeegee pressure and angle, etc., while screen printing equipment, tooling, mesh distance and squeegee width The main factors affect the positional accuracy of the printed graphics, and high-quality screen stencils are essential to improve printing accuracy.