PCB plate making methods in most PCB&PCBA manufacrurers are divided into: direct plate making method, direct and indirect plate making method, and indirect plate making method. The materials used are: sensitive paste, sensitive film, and indirect film.
1. Direct plate making method
Method: apply a certain thickness of sensitive paste (usually diazo salt sensitive paste) on the stretched screen, dry the coating, and then use the plate plate and stick it into the printing machine exposure, after development, washing and drying, it will become screen printing screen.
Technological process: photosensitive slurry preparation has stretched the net, degreasing, drying, coating, drying - exposure - development - drying - retouching - finally exposure - censorship
Degreasing: use degreasing agent to remove the oil on the screen so that the sensitive paste and screen completely glued together is not easy to take off the film.
Drying: dry moisture, avoid net cloth because temperature is too high and the tension change, its temperature should be controlled in 40 ~ 45 ℃.
Preparation of photosensitive slurry: mix photosensitizer with purified water and add it into the photosensitive slurry to mix well. Leave it for 8 hours before use.
Coating: the use of scraping groove will be sensitized pulp evenly coated on the screen, according to the coating mode is divided into automatic coating machine coating film and manual coating film, the number of coating film can be determined according to the actual situation.
Should first coated blade surface coating, white gauze purpose is to fill the gap between full, avoid air bubbles, the only coating printing surface (contact with the PCB side), the use of automatic coating machine every coating can increase the film thickness 3 um, so the coating method choices for resistance welding screen: blade surface coated twice - drying - printing surface coated three times - drying - printing surface coated three times - drying - printing surface coating, drying three times.
Description of coating method
A. The correct thickness of coating blade surface and printing surface is appropriate and meets the requirements.
B. disadvantages of thin film (printing surface) : poor durability.
C. The coating film on the scraper surface is too thick: because the sensitive paste on the scraper surface is too thick, the light is uneven. After water washing during development, rough surface ink is poured into the film layer, causing the film layer to fall off, resulting in short service life of the screen.
D. Too thin coating film on scraper surface: poor durability.
Uniform drying: dry photosensitive slurry, avoid photosensitive slurry dry wet, high temperature will let outside photosensitive slurry dry and it did not work, and make the network version of the life cut short, its temperature should be kept in 40 "45 ℃, time of 10 minutes, according to the different film thickness properly adjust the drying time.
Exposure: appropriate exposure can make the photoplasm photopolymerization, through the substrate development of a clear image.
Factors affecting the quality of screen:
A. correct exposure energy
B. Exposure and vacuum degree
C. Clean the glass of the exposure machine
General exposure energy with exposure time to adjust, the production should be based on the number of screen mesh, film thickness using hierarchical exposure method through the use of exposure measurement film to determine the correct exposure time of all kinds of screen.
Method of using the measurement piece:
(1) to estimate the exposure time in advance and then double the time for exposure, in the normal way of development, after development to choose the best effect, that is, one of the clearest image range, and then the actual exposure time times the selected image marked coefficient, that is, the better exposure time.
There are 5 coefficients on the measurement sheet, namely: 1.0, 0.7, 0.5, 0.33 and 0.25. Each coefficient corresponds to a target figure and a dot.
(2) if the coefficient 1.0 looks like the best coefficient, then to just double the exposure time, replate exposure test.
(3) such as coefficient 0.25 looks like the best coefficient, then to just reduce the exposure time in half, replate exposure test.
(4) if the continuous several coefficients are satisfied, then the dot pattern when the upper limit of the coefficient is small, that is, the exposure time is shorter, generally thicker lines when the lower limit of the coefficient is higher, that is, the exposure time is longer.
(5) such as a number of continuous coefficient are satisfied according to the type of the sun screen version, select and compare the target of the thinnest line or dot resolution select the best coefficient.
In addition, the negative of the close paste, exposure machine glass cleaning, vacuum degree on the quality of the plate has an important impact.
Development: make use of the water-soluble characteristics of photosensitive pulp, use water to wash away the unexposed photosensitive pulp, the way of development has a great impact on the fine screen, before the development of the photosensitive pulp must first water absorption solution, static 1"2 minutes, and then use the high-pressure water gun fan back and forth development, until the image is completely clear.
Note: high-pressure water washing should not be too close to the screen version, generally 0.8"1m, otherwise the pressure is too high to create jagged lines, and some dots will be washed out in case of serious problems.
Drying: will screen on moisture dry, temperature is too high, otherwise it will result in a change to screen tension, generally for 40 "45 ℃.
Revision and inspection: repair and inspect pinholes and some NPTH holes.
Final exposure: further improve the adhesion of sensitive pulp and mesh, increase the service life.
Screen sealing: fill the blank part of the screen plate with screen sealing pulp to avoid ink leakage during printing.