PCB Tech - 3 common methods and functions of PCB plate making

PCB plate making methods are divided into: direct plate making method, direct and indirect plate making method, indirect plate making method. The materials used are: photosensitive film, photosensitive film, and 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 use the plate-making film to attach it to the exposure machine for exposure, develop and rinse, After drying, it becomes a screen printing screen.

PCB process: the preparation of photosensitive paste has stretched the net-degreasing-drying-coating-drying-exposure-development-drying-revision-final exposure-sealing

The methods and functions 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 peel off.

Drying: Dry the moisture to avoid changes in the tension of the mesh due to excessive temperature. The temperature should be controlled at 40～45 degree Celsius.

Photosensitive paste preparation: mix the photosensitizer with pure water and add it to the photosensitive paste and stir evenly, and then use it after standing for 8 hours.

Coating: Use a doctoring groove to evenly coat the photosensitive paste on the screen. According to the coating method, it can be divided into automatic coating machine coating and manual coating. The number of coatings can be determined according to the actual situation.

When coating the film, the scraper surface should be coated first. The purpose is to fill up the gaps between the meshes to avoid 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 screen coating methods are: two coatings on the doctor blade surface-drying-three coatings on the printing surface-drying-three coatings on the printing surface- Drying-coating three times on the printing surface-drying.

Film coating method **Incorrect description:

A. The correct thickness of the squeegee surface and printing surface of the coating film is appropriate and meets the requirements.

B. Thin coating film (printing surface) Disadvantages: poor durability.

C. The squeegee surface coating film is too thick. Disadvantages: Because the squeegee surface photosensitive paste is too thick, the sensitivity is uneven. After the water is washed during development, the rough surface ink is poured into the film layer, which causes the film layer to fall off and shortens the screen life.

D. The squeegee coating film is too thin. Disadvantages: poor durability.

Drying: make the photosensitive paste dry uniformly, avoid the photosensitive paste from drying outside and inside being wet. Too high temperature will cause the outside photosensitive paste to dry first but not the inside, which will shorten the life of the screen. The temperature should be kept at 40"45 degree Celsius. The time is about 10 minutes, and the drying time is appropriately adjusted according to different film thicknesses.

Exposure: Appropriate exposure can photopolymerize the photosensitive paste and develop a clear image through the master.

Factors affecting the quality of the screen:

A. Correct exposure energy

B. Exposure and vacuum degree

C. Cleaning of exposure machine glass

The general exposure energy is adjusted by exposure time. In production, the correct exposure time of various screens should be determined by using the exposure measurement film by using the step exposure method according to the number of screen meshes and film thickness.

How to use the calculation film:

1. The pre-estimated exposure time is doubled for exposure, developed in a normal way, and the best effect is selected after development, that is, the clearest image range, and then the actual exposure time is multiplied by the selected image The coefficient above is the best exposure time.

There are 5 coefficients on the measurement chip, namely: 1.0, 0.7, 0.5, 0.33, 0.25, and each coefficient corresponds to a target pattern and dot.

2. If the coefficient of 1.0 seems to be the best coefficient, double the exposure time just now and remake the exposure test.

3. If the coefficient of 0.25 seems to be the best coefficient, the exposure time should be reduced by half, and the exposure test should be re-made.

4. If several consecutive coefficients are satisfied, the coefficient with a smaller upper limit is used when printing dot patterns, that is, the exposure time is shorter, and the coefficient with a higher lower limit is used when printing generally thicker lines, that is, the exposure time is longer.

5. If several consecutive coefficients are satisfied, choose the best coefficient according to the type of screen to be printed, and compare the definition of the thinnest line or dot of the target.

In addition, the adhesion 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 paste is washed away with water. The development method has a great influence on the fine screen. Before development, water must be sprayed to make the photosensitive paste absorb water and swell, and let it stand for 1" For 2 minutes, use a high-pressure water gun to develop fan-shaped back and forth until the image is completely clear.

Note: The high-pressure water washing should not be too close to the screen, usually 0.8"1m, otherwise the pressure is too large and the lines are prone to jagged, and some of the dots will be washed out in severe cases.

Drying: Dry the moisture on the screen, the temperature should not be too high, otherwise the screen tension will change, generally 40"45 degree Celsius.

Repair and inspection: repair and inspect pinholes and part of NPTH holes.

Final exposure: Further improve the adhesion of the photosensitive paste to the mesh and increase the service life.

Screen sealing: Fill the vacant part of the screen with a sealing paste to avoid ink leakage during printing.

⑵Direct plate-making method of photosensitive film

Process flow: stretched mesh-degreasing-moisturizing-filming-drying-reinforcement-exposure-development-drying-revision-sealing

method

Photosensitive film, commonly known as water film, is made of a transparent plastic film with a thickness of 0.1mm as the base and coated with a layer of photosensitive emulsion of a certain thickness on one side. The screen is fully moistened before use, and then the film is applied. The film is adsorbed to the screen by capillary action, and after drying, the plastic film base is torn off for exposure and development, and the desired pattern is finally obtained.

Wetting: Wetting is used to promote the formation of a uniform water film on the surface of the screen, so that the photosensitive film can be transferred smoothly.

Film: Stick the photosensitive film on the wet screen squeegee surface according to the screen type, and scrape off the excess water with a squeegee after sticking.

Drying: Generally, it should be dried below 40 degree Celsius. After drying, tear off the plastic base and let it dry for a few more minutes.

Reinforcement: If you need to increase the number of printing plates, you can apply a layer of photosensitive paste on the doctor blade after the film is dried, and it needs to be dried after coating.

The other part of the operation is consistent with the photosensitive paste plate making method.

2. Indirect plate making method

method

The method of indirect plate making 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 surface of the graphic film is tightly attached to the stretched screen. Squeeze to make the glue film adhere to the wet screen, remove the film base, and dry it with air to form a screen printing screen.

Process flow:

1. Stretched net-degreasing-drying

2. Indirect film-exposure-hardening-development

1and2--fitting--blowing-drying--revision--screening

3, direct and indirect plate making method

method

The method of direct and indirect plate making is that the PCB factory first puts the photosensitive film coated with photosensitive material on the work surface on the work surface, and then puts the stretched wrist screen frame on the film base, and then puts it in the screen frame. Put the photosensitive paste and apply pressure with a soft squeegee. After fully drying, the plastic film base is removed, and the photosensitive film wrist screen can be used for printing. After development and drying, a screen printing screen is produced.

Process flow:

has stretched the net-degreasing-drying-peeling off the base-exposure-development-drying-revision-sealing