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When discussing the development trend of printed circuit board (PCB) technology, we noticed is moving towards high precision, high density, and high reliability. However, at the same time, the appearance quality standards of PCBs are becoming more and more stringent. For example, the solder resist coating is not only required to have a certain thickness and hardness, and pass the solvent resistance and adhesion tests, but also must have a uniform surface color and gloss, and no impurities or excess traces.
The appearance quality of solder resists reflects the technical level and management level of the enterprise and has a direct impact on the production order of the enterprise. Therefore, improving the appearance quality of solder resists on printed circuit boards has become a key issue that PCB manufacturing plants must solve urgently.
1. Printing process: During the screen printing process of photosensitive solder resist ink, factors such as the flatness of the blade, the degree of environmental purification, the use of barrier tape, and the ink printing pressure will affect the appearance quality. In actual production, the first three factors are particularly important. If the blade is not flat, it is easy to leave marks on the solder resist printing surface; improper environmental purification is easy to causes impurities to adhere; improper use of tape may cause solvents and surface particles in the ink to mix.
2. Exposure process: During the contact process of welding ink, if the flux is not completely cured, the solder resist layer and welding paste make it easy to produce marks, which is the main factor affecting the appearance quality of the welding paste. During the development process, if the solder resist is not completely cured, the driving wheel and pressure wheel of the developer and other parts may cause surface damage, forming roller marks, which in turn affects the appearance quality of the solder resist.
3. Post-curing treatment: If the welding temperature is unevenly distributed after curing, it is easy to cause uneven welding paste color. Excessive temperature may even cause local discoloration, affecting the appearance. When screen printing welding ink, due to the uneven screen printing surface, the scraper surface will become uneven after long-term use, leaving scratches on the solder resist surface.
Therefore, the operator must always pay attention to the surface condition. Once the scraper marks are found, they should be cleaned immediately to ensure the smoothness of the solder resist. To obtain printed boards with excellent appearance quality, the cleanliness of the screen printing room is essential. All parts in contact with the printed circuit board (including the workbench, screen frame, blotting paper, and solder resist tape) and the printed circuit board must be dusted with roller dust. During the turnover process, it is necessary to ensure the cleanliness of the room, wear special work clothes, and hats, and comply with the wind bath regulations. In addition, air purification protection measures around the factory are also extremely important, which can be achieved by regular water spraying and dust removal.
First, the drive roller must use soft materials or roller coatings, such as soft PVC "O" rings or squeeze rollers made of soft rubber. Second, ensure the stability of the entire transmission system. Finally, regularly clean the developer roller and squeeze the roller to remove dirt on the roller and prevent roller marks.
To improve the appearance quality of the PCB solder resist, comprehensive control must be carried out from multiple aspects such as process, raw materials, equipment, and operators, especially strict control of screen printing parameters, exposure and development, and curing. Through these measures, the appearance quality of the solder resist layer of the printed circuit board will be able to meet customer needs.