Faults in The Screen Version During PCB Screen Printing

The faults in screen printing include problems in screen printing and screen printing process. This article only describes the causes and countermeasures of screen printing failures.

1.1 Plugging Holes
When the new screen is used for the first time, the printing material is too bad, mainly due to the quality of the plate. It may be due to the residual glue on the mask. The screen can be scrubbed with a solvent to remove the residual glue. If it still cannot be used, the screen can only be remade. If the screen printing performance is good at the beginning of the screen printing, and the printing material coating gradually becomes thinner and pitted during the screen printing process, the screen printing material cannot be printed at all. The reasons for this may be that the printing material dries too fast, the screen dries up and blocks the holes, the printing speed is too fast, the hardness of the scraper is too high, etc. In this regard, a solvent with slow volatility should be used to adjust the printing material, and soft cloth dipped in the solvent should be used to gently scrub the screen. After the screen printing, the screen must be cleaned with a solvent, otherwise, the residual printing material will dry up and block the mesh.

1.2 Screen Ink Leakage

During screen printing, the printing material on the screen drips onto the printed board. The reasons for this failure are that there is dust and debris on the surface of the printed board or the printing material, the screen is damaged due to the pressure of the scraper, dust or debris during screen printing. Insufficient exposure of the glue, resulting in incomplete curing of the screen mask, falling off during screen printing to form small holes, resulting in ink leakage. In this regard, tape or tape can be used to stick on the screen holes, and screen glue can also be used to repair them.

1.3 Broken Screen And Reduced Accuracy

After a long-term use of the screen, due to the abrasion and printing pressure of the scraper, even the screen of very good quality, its accuracy will gradually decrease or be damaged. This is determined by the life of the screen version. The number of times of printing (or the length of life) of the screen is closely related to the method of making the mask, and the life of the direct method screen is longer than that of the indirect method screen. Under normal conditions, the direct French screen can be printed 20,000 to 30,000 times. Generally speaking, mass production adopts direct method screen plate. Direct plate making adopts water-soluble photosensitive resin emulsion with good water and solvent resistance after curing. Due to the humid air in the rainy season, it is easy to affect the quality and service life of direct legal plate making. The damage of the indirect method screen is not to peel off the mask and the screen at once, but to peel off the screen bit by bit due to erosion. The direct and indirect method is used to make the screen, the thickness of the masking film is uniform and the thickness can be set, so the edge of the image is clear, but the shrinkage of the film and the screen edge is inconsistent during the heating process. If there is dust between the screen and the photosensitive film when applying the film, it will cause poor adhesion between the photosensitive film and the screen, affecting the quality and life of the screen printing. The countermeasure is to wipe it with an anti-static cloth before applying the film.

1.4 Failure Caused by Excessive Printing Pressure

During screen printing, the force applied to the squeegee produces a printing pressure, the purpose of which is to make the screen plate and the printing plate in line contact, and the squeegee scrapes the printing material to form an image through the screen. The size of the printing pressure depends on the screen tension, the length of the squeegee, and the distance between the screen and the printed board (screen distance). The high pressure of the squeegee will not only make the printing material penetrate a large amount, but the printing pressure will be too large, causing the squeegee to bend and deform, but will reduce the printing material penetration, and even make the screen and the printed board not in line contact but surface contact. Not only can it not print a clear image, but also cause scraper wear and screen mask shedding, screen elongation, and image deformation.