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, slow solvent evaporation media should be used to gently scrub the screen with a soft cloth. After the screen is printed, the screen must be cleaned with solvent, otherwise the residual ink will clog the screen. .
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, or it can be repaired with a 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 because of the 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. With the direct and indirect method of making screens, the thickness of the masking film is uniform and the thickness can be set. Therefore, the edges of the mask are clear, but the film film and the edges of the screen are inconsistent during the heating process. The countermeasure is to coat the direct photoresist with the photosensitive film around the film. 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 the screen printing quality and life. The countermeasure is to wipe the anti-static cloth before applying 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 will not only cause a large amount of printed material to flow, but also cause too much pressure, causing the scraper to be bent and deformed. This will result in a reduction in the amount of printed material, and even the surface of the screen and the printed circuit board will not be in contact with each other. Not only can you not print clear images, but it can also cause scrape wear and screen mask peeling, web stretching, and image distortion.
3. Printed board related failures
3.1 poor adhesion
Poor bonding not only has factors in the printed materials, but also causes the printed board. Poor surface treatment of the printed board is an important cause of poor bonding. Different processing methods must be adopted according to the characteristics of various materials.
3.2 Inaccurate Position of Image Printing
Even if there are no problems in the screen size, screen printer, etc., the printed board material does not have the same shape, and the material shrinkage is too large and inconsistent, which causes failure. When the printing material is a flexible material, such as a plastic film, changes in the temperature of the screen printing place can cause changes in its size, affect the accuracy of screen printing, screen printing must be taken into account, as much as possible in advance calculations to give correction. Take corresponding measures.
3.3 Precision Graphic Printing
Screen printing, printed circuit boards, thick-film ICs, resistors, etc. require high screen-printing accuracy, and thus the temperature of the network frame structure, stretch netting, screen making, printing, screen printing, and screen printing environments. Humidity, cleanliness, as well as the performance of the printing materials themselves and the surface treatment have very strict requirements.
4. Manual screen printing general failures and countermeasures
1) Before the screen printing, the nature of the printed board material should be further understood and confirmed.
2) Check the screen carefully before screen printing.
3) After the screen printing is completed, the screen plate must be cleaned with a solvent and no printed material can be left. Otherwise, it will cause plugging.
4) Do not use resin tape to stick to the screen when it is a last resort, because sticking with a resin tape will cause the screen to shrink, resulting in errors in the size of the pattern.
5) If it is necessary to stick the tape, try not to stick the tape near the figure. If the tape is placed too close to the picture, it is easy to produce spots when screen printing.
6) In the process of screen printing, it is necessary to pay attention to the change of the state of the printed material. If foreign matter is mixed into the printed material, the foreign matter should be removed before printing.
7) The scraper is easily affected by the solvent, so after the screen printing, the scraper must be cleaned. In addition, it is better to use several scrapers for rotation. This can not only prolong its service life, but also have a comparative advantage. Good screen printing.
8) During screen printing, carefully inspect the squeegee. If the squeegee has wear or gaps, grind the squeegee.
(a) (b) (c) (d) (e)