Analysis and Treatment of Common Faults in PCB Screen Printing

1.6 screen printed wire sawtooth

Screen printed conductors are serrated. The reason for this phenomenon is that when making a screen, the mask is too thin, and the screen image itself has a gap. To avoid this phenomenon, the plate mask must be of appropriate thickness and elasticity, and the high-required image should be made of polyester mesh with a high mesh. The direction of the screen printing wire pattern should be as consistent as possible with the direction of scraping. If the wire is in the direction of the vertical squeegee, it is very easy to produce this phenomenon. In addition, when the net is stretched, the wire direction of the wire mesh is at a certain angle with the frame, and the best angle is 22.5°.
Countermeasures:

A. Produce high-quality screen images. The edges of screen lines must be stiff.
B printed board adsorption of printed materials is better.
C printing thixotropic is better
The distance between the D screen plate and the printed board and the angle of the scraper blade should be appropriate.
E. The screen tension and screen pitch should be appropriate.

1.7 screen printing patterns on the screen pattern

The screen printing pattern on the screen printing pattern is mainly because the selected screen mesh is too low, the thixotropy of the printing material is not good, and the drying and curing speed of the printing material is too fast. For this purpose, high-mesh screens can be used to make the screens, with good thixotropic properties and dry and slow printing materials.

1.8 spots

Screen printing patterns appear spotted phenomenon and affect the smooth screen printing. The reasons for this phenomenon are:
1.8.1 Screen printing speed and drying of printing materials are too slow
1.8.2 Insufficient solids content
1.8.3 The thixotropy of the printed material is too large
1.8.4 Effect of static electricity
1.8.5 The pigments in the printed material are not uniformly dispersed, and the pigment particles attract and agglomerate to cause color spots.

Countermeasures:

(1) improve the flow of printed materials,

(2) Use a quick solvent with a high volatilization rate,

(3) Use high viscosity printing materials,

(4) Minimize static electricity as much as possible, and static elimination devices may be used.

1.9 uneven color screen printing graphics

The reasons for the uneven screen printing color are various, and in terms of printing materials, the colorants in the printing materials are unevenly distributed, the fineness of the printing material is large, and the printing ink is blotted. To prevent such failures, filter the printed material before screen printing, and fully stir the printed material.

2. Screen failure

The screen troubles include the problems caused by the screen editions and the problems caused by the screens in the screen printing process. This article only describes the causes of the screen troubles in the screen printing process and the countermeasures.

2.1 Holes

When the new screen version was first used, the printing material was poor, mainly due to the quality of the platemaking. It may be due to masks remaining on the screen. You can use the solvent to scrub the screen plate to remove excess adhesive. If you still cannot use it, you must re-do the screen printing. If screen printing starts with screen printing, the screen coating will gradually thin and pits, and printing will not occur. The reasons for this may be: the printing material is drying too fast, the blocking hole is dried in the screen plate, the printing speed is too fast, and the scraper hardness is too high. In this regard, a solvent with low volatility should be used to adjust the printing material, and the screen plate should be gently scrubbed with a solvent on a soft cloth. After the screen printing is completed, the screen plate must be cleaned with a solvent, otherwise the residual printing material will clog the mesh. .

2.2 screen printing ink

During screen printing, the printing material on the screen dripped onto the printed board. The reasons for this failure are: damage to the screen due to scraper pressure, dust, or sundries when there is dust, debris, or miscellaneous materials on the surface of the printed board or in the printed material; in addition, it may be due to masking during screen making. Inadequate exposure of the glue causes the screen mask to be incompletely cured, and the screen is peeled off to form small holes, resulting in ink leakage. In this regard, adhesive tape or tape can be applied to the screen holes, and can also be repaired with screen printing paste.

2.3 Screen damage and precision degradation

After long-term use of the screen, the accuracy of the screen will be gradually reduced or damaged even if the screen is of good quality due to abrasion and pressure of the plate. This is determined by the life of the screen. The number of prints (or the length of life) of the screen is closely related to the method of plate-making. The life of the direct screen is longer than that of the inline screen. Under normal conditions, the direct screen version can withstand 2 to 30,000 prints. In general, mass production uses direct legal screens. The direct process plate uses a water-soluble photosensitive resin emulsion having good water resistance and solvent resistance after curing. In the rainy season, due to the dampness of the air, the quality and service life of the direct legal plate are easily affected. The damage of the indirect screen plate was not peeled off from the screen and it was peeled away from the screen by erosion. The direct and indirect method of making screen plate, mask thickness and thickness can be set uniform, so the edge of the image is clear, but in the heating process film film and the edge of the screen shrink is not consistent, the solution is to apply a direct legal system around the photosensitive adhesive When covering the film film, if there is dust in the middle of the screen and the photosensitive film, it will cause poor adhesion between the partial photosensitive film and the screen, which will affect screen printing quality and service life. The countermeasure is to wipe the film with antistatic cloth before sticking the film. Diaphragm.

2.4 Printing pressure caused by excessive failure

During screen printing, the force applied to the squeegee produces a press pressure, the purpose of which is to make the screen plate in line contact with the printed board, and the squeegee scrapes the printed material to form an image through the screen. The size of the press pressure depends on the screen tension, the length of the squeegee, the distance between the screen and the printed board (net distance). The high pressure of the scraper not only results in a large amount of printed material, but also a large printing pressure, which causes the scraper to be bent and deformed, which in turn reduces the throughput of the printed material, and even makes the screen and the printed board not in contact with each other and become surface contact. Not only can you not print clear images, but it can also cause scrape wear and screen mask peeling, web stretching, and image distortion.

(to be continued)

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