PCB plate making methods are divided into direct plate making method, direct indirect plate making method and indirect plate making method. The materials used are: photosensitive paste, photosensitive film, photosensitive film and indirect film

1. Direct plate making method

Methods: apply a certain thickness of photosensitive paste (usually diazonium salt photosensitive paste) on the stretched screen, dry it after coating, then fit it with the plate making negative, put it into the plate printer for exposure, and become a screen printing screen after development, washing and drying.

Process flow: photosensitive slurry preparation, stretched screen – degreasing – drying – Coating – drying – exposure – Development – drying – Revision – final exposure – screen closure

Methods and functions of each section

Degreasing: degreasing agent is used to remove the grease on the screen, so that the photosensitive slurry and the screen are completely glued together, so that it is not easy to take off the film.

Drying: dry the moisture level to avoid the tension change of the mesh cloth due to too high temperature, and the temperature shall be controlled at 40 ~ 45 ℃.

Preparation of photosensitive paste: mix the photosensitizer with purified water, add it into the photosensitive paste, stir it evenly, and reuse it after 8 hours.

Coating: the photosensitive paste is evenly coated on the screen by the scraper. It is divided into automatic coater coating and manual coating according to the coating mode. The coating times can be determined according to the actual situation.

When coating, the scraper surface shall be coated first, in order to fill the ground gap between the gauze to avoid bubbles, and then the printing surface (the side in contact with PCB) shall be coated. At present, the automatic coating machine can increase the film thickness by about 3um every time, Therefore, the coating method of solder mask is mostly selected as: scraper surface coating twice – drying – printing surface coating three times – drying – printing surface coating three times – drying – printing surface coating three times – drying.

Three methods and process flow analysis of PCB plate making

Incorrect description of coating party * *:

A. The thickness of correct film scraper 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 photosensitive slurry on the scraper surface is too thick, the photosensitivity is uneven. During development, after water washing, the surface is rough, and the ink is poured into the film layer, so that the film layer falls off, resulting in the reduction of screen life.

D. The coating film on the scraper surface is too thin. Disadvantages: poor durability.

Drying: dry the photosensitive paste evenly to avoid drying outside and wet inside. Too high temperature will make the outside photosensitive paste dry first and the inside not dry, which will shorten the service life of the screen. The temperature should be kept at 40 “45 ℃ for about 10 minutes. The drying time should be adjusted appropriately according to different film thickness.

Exposure: proper exposure can make the photosensitive paste photosensitive polymerization, and develop a clear image through the base plate.

Factors affecting screen quality:

A. Correct exposure energy

B. Exposure and vacuum degree

C. Cleaning of exposure machine glass

Generally, the exposure energy is adjusted by the exposure time. During the production, the graded exposure method shall be adopted according to the mesh number and film thickness of the screen plate, and the correct exposure time of various screen plates shall be determined by using the exposure measuring film.

How to use the tablet:

① The exposure is performed with the pre estimated exposure time plus twice the time, and developed in a normal way. After development, the best effect, that is, one of the clearest image ranges, is selected, and then the actual exposure time is multiplied by the marked coefficient on the selected image, which is the better exposure time.

It is estimated that there are 5 coefficients on the chip, namely: 1.0, 0.7, 0.5, 0.33 and 0.25. Each coefficient corresponds to a circular target pattern and dot respectively.

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

③ If the coefficient of 0.25 seems to be the best coefficient, reduce the exposure time by half and retest the plate making exposure test.

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

⑤ If several consecutive coefficients are satisfied, select the best coefficient by comparing the sharpness of the thinnest line or dot of the circular target according to the type of screen plate.

In addition, the close adhesion of the negative, the glass cleaning of the exposure machine and the vacuum degree all have an important impact on the print quality.

Development: make use of the water solubility of the photosensitive slurry and wash the unexposed photosensitive slurry with water. The development method has a great impact on the fine screen. Before development, spray water to make the photosensitive slurry absorb water and dissolve first, stand for 1 “2 minutes, and then develop back and forth in a fan with a high-pressure water gun until the image is completely clear.

Note: high pressure water washing should not be too close to the screen, generally 0.8 “1m, otherwise the pressure is too high, the lines are easy to produce sawtooth, and some dots are washed out in serious cases

Drying: dry the water on the screen, and the temperature should not be too high, otherwise the screen tension will change, generally 40 “45 ℃.

Revision and inspection: repair and inspect pinholes and some npth holes.

Final exposure: further improve the adhesion between photosensitive paste and screen yarn and increase the service life.

Screen sealing: fill the empty part of the screen with screen sealing slurry to avoid ink leakage during printing.

⑵ photosensitive diaphragm direct plate making method

Process flow: stretched Net – degreasing – wetting – film coating – drying – reinforcement – exposure – Development – drying – Revision – screen sealing

method

The photosensitive film, commonly known as water film, is based on a transparent plastic film with a thickness of 0.1mm and coated with a layer of photosensitive emulsion with a certain thickness on one side. When in use, the screen is fully wetted, and then the film is pasted. The film is adsorbed to the screen through capillary action. After drying, the plastic film base is torn off for exposure and development, and finally the required graphics are obtained.

Wetting: wetting is used to promote the formation of a uniform water film on the surface of the screen, so as to make the photosensitive film transfer smoothly.

Film: stick the photosensitive film on the wet screen scraper surface according to the screen type, and scrape off the excess water with a scraper after pasting.

Drying: generally, it should be dried below 40 ℃. After drying, tear off the plastic sheet base and dry it 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 scraper surface after the diaphragm is dry, and it needs to be dried after coating.

The operation of other parts is consistent with the photosensitive pulp plate making method.

2. Indirect plate making

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 strippable graphic negative. During plate making, stick the adhesive film surface of the graphic negative with the stretched screen, press the adhesive film with the wet screen, remove the film base, and dry it with air to make a silk screen.

technological process:

1. Stretched Net – degreasing – drying

2. Indirect film – exposure – hardening – Development

1and2 – fit – blow dry – Revision – screen closure

3. Direct and indirect plate making

method

The method of direct and indirect plate making is to first lay the wrist film base coated with photosensitive material flat on the worktable, put the stretched wrist mesh frame flat on the film base, then put photosensitive slurry in the mesh frame and apply it under pressure with a soft scraper, remove the plastic film base after sufficient drying, and attach the wrist mesh with photosensitive film, which can be used for plate printing after development After drying, silk screen printing is made.

technological process:

Stretched screen – degreasing – drying – stripping film base – exposure – Development – drying – Revision – screen closure

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