People talk about the trend in the development of printed circuit board, printed circuit board that tend towards high precision, high density and high reliability of the direction of development, this is the trend of development. But on the other hand, the appearance of PCB is becoming more and more strict. Like the printed circuit board solder resist "jacket", in addition to the requirements of the certain thickness and hardness, solvent resistance test and adhesion test standard, but also for the surface of uniform color and shiny surface, no garbage, no extra mark. It can be said that the appearance quality of solder resist is not only the embodiment of the technical level and management level of an enterprise, but also directly affects the order of an enterprise". Therefore, how to improve the appearance quality of printed circuit board solder resist has become a problem to be solved for each PCB factory.
Factors affecting appearance quality of solder resist
1, printing: screen printing process of photosensitive solder resist ink, blade flatness, environmental purification, screen printing screen printing using block tape and ink printing pressure, printing preparation before printing plate would affect the appearance quality. According to the actual situation of production, the most influential factors are the first three. The blade is not easy to produce flat blade mark on the surface of solder resist printing; between the degree of purification is easy to resist in the surface of garbage; improper use of block tape, easy to make the gum in ink solvent and surface particles.
2. Exposure: in the process of exposure to resist welding ink, because the solder flux is not completely cured, the solder mask and solder paste together easily produce imprint, which is the main reason affecting the appearance quality of solder paste.
3, developing: the solder resist ink by developing a general level transfer type developing, the solder resist is not completely cured, developing machine driving wheel, pressure wheel and so easy to cause damage to the surface, produce roller mark, thus affecting the appearance quality of solder resist. In addition, the incorrect exposure energy also affects the gloss of solder resist, but this can be controlled by the wedge table.
4, after curing: resistance to solder after curing temperature is uneven, easy to cause solder paste color is uneven, when the temperature is too high and even cause local yellowing, black, affect solder appearance.
When the screen printing solder ink, because the screen printing surface is uneven, after a period of screen printing, the scraper surface will become uneven, so in the solder resist surface will leave a scraper mark. Therefore, the operator must pay attention to observe the surface condition at any time, once found the scraper mark, should immediately rub the scraper, in order to ensure its smoothness.
In order to obtain the printed board with good appearance quality, the cleaning degree of the screen printing room plays a key role. All with the printed circuit board contact place (including table, screen frame, blotting, BLOCKOUTS tape) and printed circuit board itself to dust with roller dust, the turnover must be clean and clean room, clean room into the operator must wear special clothing, wearing a cap, and according to the provisions of the wind bath. At the same time, the protection of air purification around the factory is also essential. If the condition, can regularly sprinkle water around the factory for dust removal.
Development: the solder flux is not completely solidified on the developing surface, and it is easy to leave the impression of rollers. Therefore, it should be considered on the equipment. The first driving roller must use soft materials or in roller coat, soft PVC "O" ring, roller, a water squeezing roller made of soft rubber roller; secondly to ensure the stability of the whole transmission system; finally, on a regular basis to the developing roller and a water squeezing roller for cleaning the dirt stick the roller removed, can produce such anti roll mark.
To improve the appearance quality of the PCB solder mask, process discipline from process, raw materials, equipment, operators and other aspects of comprehensive control, especially for the parameters of screen printing, exposure and development, after curing the strict control. In this way, the solder resist appearance quality of the printed circuit board can completely satisfy the customers.