Comparative Analysis of Three Screen Printing Methods
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Classification of screen production
Plate making method using materials
Direct plate making method
Direct indirect plate making method
Indirect plate making method
1. Direct plate making method
(1) Photosensitive pulp direct plate making method
Method: Applying a certain thickness of the photosensitive paste (usually diazonium salt photosensitive paste) on the stretched screen, drying after coating, and then using the plate-making negative film to fit it into the printing machine for exposure, developing and rinsing After drying, it becomes a screen printing screen.
Process flow: photosensitive paste preparation has been stretched - degreasing - drying - coating film - drying - exposure - development - drying - revision - final exposure - sealing network
Methods and functions of each section
Degreasing: The degreasing agent is used to remove the grease on the screen to completely glue the photosensitive paste and the screen together to remove the film.
Drying: Drying the moisture to prevent the mesh from changing due to excessive temperature. The temperature should be controlled at 40 to 45 °C.
Preparation of photosensitive paste: Mix the photosensitizer with pure water, add it to the photosensitive slurry, stir evenly, and use it after 8 hours.
Coating film: The photosensitive paste is evenly coated on the screen by a scraping groove, and is divided into an automatic coating machine coating film and a manual coating film according to the coating film method, and the number of coating films can be determined according to actual conditions.
When applying the film, the scraper surface should be applied first. The purpose is to fill the gap between the meshes first to avoid the generation of bubbles. Then the coated surface (the side in contact with the PCB) is applied, and the automatic coating machine currently used is coated. The film thickness can be increased by 3um at a time, so most of the solder resist screen coating methods are: the blade surface is coated twice - drying - printing surface coating three times - drying - printing surface coating three times - Drying - printing surface coated three times - drying.
The film coating method is incorrectly stated:
A. The correct coating blade surface and printed surface thickness are suitable and meet the requirements.
B. Thin coating film (printing surface) Disadvantages: Poor durability.
C. The coating surface of the doctor blade is too thick. Disadvantages: Because the photosensitive surface of the doctor blade is too thick, the photosensitive is uneven. After the water is rinsed during development, the rough surface ink is poured into the film layer, causing the film layer to fall off, resulting in a shortened life of the screen.
D. The scraper surface coating film is too thin. Disadvantage: poor durability.
Drying: Make the photosensitive paste dry evenly, avoiding the wetness inside the photosensitive paste. If the temperature is too high, the external photosensitive paste will dry first and the inside will not dry, and the life of the screen will be shortened, and the temperature should be kept at 40~45 °C. The time is about 10 minutes, and the drying time is appropriately adjusted according to different film thicknesses.
Exposure: Proper exposure allows the photosensitive paste to be photopolymerized and a clear image developed through the master.
Factors affecting the quality of the screen:
A. Correct exposure energy
B. Exposure and vacuuming
C. Cleaning of the exposure machine glass
The general 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 measurement method according to the mesh number and the film thickness.
How to use the test piece:
1 Exposing with a pre-estimated exposure time and doubling the time, developing in the normal way, selecting one of the best-performing image areas after development, and then multiplying the actual exposure time by the selected one. The coefficient marked on the image is the preferred exposure time.
There are five coefficients on the measurement chip, namely: 1.0, 0.7, 0.5, 0.33, and 0.25. Each coefficient corresponds to a circle target and a dot.
2 If the coefficient of 1.0 seems to be the best factor, then double the exposure time and re-plate the exposure test.
3 If the coefficient 0.25 seems to be the best coefficient, then reduce the exposure time by half, and re-plate the exposure test.
4 If several consecutive coefficients are satisfied, the upper limit coefficient is adopted when the dot pattern is printed, that is, the exposure time is shorter, and the lower limit coefficient is adopted when the drying is generally thicker, that is, the exposure time is longer.
5 If several consecutive coefficients are satisfied, select the best coefficient for comparing the sharpness of the finest line or dot of the target according to the type of the screen.
In addition, the closeness of the film, the cleaning of the glass of the exposure machine, and the degree of vacuum have an important influence on the quality of the printing plate.
Development: Using the water-soluble characteristics of the photosensitive paste, the unexposed photosensitive slurry is washed away with water, and the development method has a great influence on the fine screen. Before the development, the water must be sprayed to make the photosensitive slurry first absorb and swell, and let it stand 1~ After 2 minutes, use a high pressure water gun to fan back and forth until the image is completely clear.
Note: High-pressure washing can not be too close to the screen, generally 0.8~1m, otherwise the pressure is too large, so the line is easy to produce jagged, and some outlets are washed away in serious cases.
Drying: Dry the water on the screen, the temperature should not be too high, otherwise the tension of the screen will change, generally 40~45 °C.
Revision and inspection: Repair and inspect the pinhole and some NPTH holes.
Final exposure: further improve the adhesion of the photosensitive paste to the mesh and increase the service life.
Sealing the net: Fill the spare part of the screen with the netting paste to avoid leakage of ink during printing.
(2) Photosensitive film direct plate making method
Process: stretched net - skim - wet - film - drying - reinforcement - exposure - development - drying - revision - sealing network
Method
Photosensitive film, commonly known as water film, is made of a transparent plastic film with a thickness of 0.1mm. The enamel is coated with a layer of sensitizing emulsion on one side. When used, the screen is fully wetted and then coated. The film is adsorbed onto the screen by capillary action, and after drying, the plastic substrate is peeled off for exposure and development, and finally the desired pattern is obtained.
Wetting: The use of wetting promotes the formation of a uniform film of water on the surface of the screen to facilitate the smooth transfer of the film.
Film: Apply the film to the wet screen scraper surface according to the type of screen. After attaching it, scrape off the excess water with a spatula.
Drying: Generally, it should be dried below 40 ° C. After drying, the plastic base is peeled off and allowed to dry for a few more minutes.
Reinforcement: If you need to increase the number of plates, apply a layer of photographic paste on the squeegee after the film is dried. Dry it after coating.
The other parts of the operation are consistent with the photographic paste making method.
2, direct indirect plate making
Method
The method of direct indirect plate making is to first place the photosensitive film coated with the photosensitive material on the surface of the work surface, and put the stretched wrist frame on the base, and then put it in the frame. The photosensitive paste is pressure-coated with a soft blade, and after drying, the plastic film base is removed, and the photosensitive film wrist wire mesh is attached to the printing plate, and after development and drying, a screen printing screen is produced.
Process flow:
Stretched net - skim - drying - peeling film base - exposure - development - drying - revision - sealing net
3. Indirect plate making method
Method
The indirect plate making method is to expose the indirect film first, harden it with 1.2% H2O2, develop it with warm water, and dry it to make a peelable graphic film. When making the plate, the film film surface is tightly attached to the stretched wire mesh. Squeeze the film with the wet wire, peel off the base, and blow it dry to make a screen printing screen.
Process flow:
1. Stretched net - skim - drying
2. Indirect film - exposure - hardening - development
1and2 - fit - blow dry - revision - seal net