PCB plate making methods are divided into direct plate making method, direct indirect plate making method, indirect plate making method. The materials used are photosensitive paste photosensitive film, photosensitive film, indirect film.
1. Direct Plate Making Method
Method: Coat a certain thickness of photosensitive paste (usually diazonium salt photosensitive paste) on the stretched screen, dry it after coating, and then attach it with a plate-making negative and put it into a printing machine for exposure, and then develop and rinse. After drying, it becomes a screen printing screen.
Technological process: preparation of photosensitive paste, stretched net – degreasing – drying – coating film – drying – exposure – developing – drying – revision – final exposure – sealing net
The method and function of each section:
Degreasing: Use a degreasing agent to remove the grease on the screen, so that the photosensitive paste and the screen are completely glued together, so that it is not easy to release the film.
Drying: Dry the moisture to avoid the tension change of the mesh due to excessive temperature, and the temperature should be controlled at 40-45°C.
Preparation of photosensitive paste: Mix the photosensitizer with pure water, add it into the photosensitive paste, stir evenly, and leave it for 8 hours before use.
Coating: The photosensitive paste is evenly coated on the screen by means of a scraper. According to the coating method, it is divided into automatic coating machine coating and manual coating. The number of coating times can be determined according to the actual situation.
When coating the film, the scraper surface should be coated first. The purpose is to fill the gap between the meshes first to avoid the generation of air bubbles, and then coat the printing surface (the side that is in contact with the PCB). The film thickness can be increased by about 3um at a time, so most of the solder mask coating methods are: coating on the blade surface twice – drying – coating on the printing surface three times – drying – coating on the printing surface three times – Drying – coating the printed surface three times – drying.
A. The correct thickness of the coating scraper surface and printing surface is suitable and meets the requirements.
B. Disadvantages of thin coating film (printing surface): poor durability.
C. The coating film on the blade surface is too thick. Disadvantages: Because the photosensitive paste on the blade surface is too thick, the light sensitivity is uneven. After the water is rinsed during development, the rough surface ink is poured into the film layer, which makes the film layer fall off, resulting in a shortened screen life.
D. The coating film on the blade surface is too thin. Disadvantages: poor durability.
Drying: Make the photosensitive paste dry evenly, avoid the photosensitive paste from being dry on the outside and wet on the inside. If the temperature is too high, the photosensitive paste on the outside will dry first while the inside is not dry, which will shorten the life of the screen. The temperature should be kept at 40″45℃. The time is about 10 minutes, and the drying time is appropriately adjusted according to different film thicknesses.
Exposure: Appropriate exposure can make the photosensitive paste photopolymerize, and develop a clear image through the negative plate.
Factors affecting the quality of the screen version:
A. Correct exposure energy
B. Exposure and vacuum level
C. Cleaning of exposure machine glass
Generally, the exposure energy is adjusted by the exposure time. In the production, the correct exposure time of each type of screen should be determined by using the exposure measuring sheet by using the graded exposure method according to the screen mesh and film thickness.
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