CB board making methods are divided into: direct board making method, direct indirect board making method, and indirect board making method The materials used are photosensitive paste, photosensitive film, and indirect film

1. PCB direct method

Method: Apply 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 be attached to it and exposed in the plate making machine. After development, rinsing, and drying, it becomes a screen printing screen.

Process flow: Preparation of photosensitive paste with stretched mesh - degreasing - drying - coating - drying - exposure - development - drying - plate repair - final exposure - screen sealing

Methods and functions of each section

Degreasing: Using degreasing agents to remove grease from the screen, so that the photosensitive paste and screen are completely bonded together to prevent film detachment.

Drying: Dry the moisture and avoid tension changes in the mesh due to high temperature. The temperature should be controlled at 40-45 ℃.

Preparation of photosensitive paste: Mix the photosensitizer with pure water and add it to the photosensitive paste, stir evenly, and let it sit for 8 hours before use.

Coating: Use a scraper groove to evenly apply the photosensitive paste onto the screen, which can be divided into automatic coating machine coating and manual coating according to the coating method. The number of coating times can be determined according to the actual situation.

When coating, the first step is to apply the scraper surface to fill the gaps between the mesh and prevent the formation of bubbles. Then, apply the printing surface (the side in contact with the PCB). Currently, the automatic coating machine can increase the film thickness by about 3um per coating. Therefore, the coating method for solder mask screens is mostly selected as follows: scraper surface coating twice - drying - printing surface coating three times - drying - printing surface coating three times - drying - printing surface coating three times - drying.

Explanation of coating error:

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

B. Disadvantages of thin coating (printing surface): poor durability.

C. Disadvantages of too thick coating on the scraper surface: Due to the thick photosensitive paste on the scraper surface, the sensitivity is uneven. After rinsing with water during development, the surface becomes rough and ink is injected into the film layer, causing it to peel off and shortening the lifespan of the screen.

D. The disadvantage of a thin coating on the scraper surface is poor durability.

Drying: Dry the photosensitive paste evenly, avoiding dryness on the outside and moisture on the inside. If the temperature is too high, the outside photosensitive paste will dry first and the inside will not dry, which will shorten the lifespan of the screen. The temperature should be maintained at 40 "45 ℃ for about 10 minutes, and the drying time should be adjusted appropriately according to different film thicknesses.

Exposure: Proper exposure can cause the photosensitive paste to undergo photosensitive polymerization, resulting in clear images through the development of the base plate.

Factors affecting the quality of screen printing:

A. Correct exposure energy

B. Exposure and vacuum degree

C. Cleaning of exposure machine glass

The general exposure energy is adjusted by the exposure time. In production, the graded exposure method should be used based on the mesh size and film thickness of the screen to determine the correct exposure time for various types of screens by using an exposure measurement film.

PCB fabrication

Usage method of calculation sheet:

① Expose the image using the pre estimated exposure time plus double the time, develop it in the normal way, and select the best effect, that is, the clearest image range after development. Then multiply the actual exposure time by the coefficient marked on the selected image to obtain the optimal exposure time.

There are a total of 5 coefficients calculated on the chip, namely 1.0, 0.7, 0.5, 0.33, and 0.25, each corresponding to a circular target shape and dot.

② If a coefficient of 1.0 appears to be the optimal coefficient, then the exposure time should be doubled and a new plate exposure test should be conducted.

③ If a coefficient of 0.25 appears to be the optimal coefficient, then the exposure time should be reduced by half and a new exposure test should be conducted.

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

⑤ If several consecutive coefficients are satisfied, the optimal coefficient is selected based on the type of screen being sun dried, comparing the clarity of the thinnest lines or dots on the circular target.

In addition, the sealing of the film, the cleaning of the exposure machine glass, and the vacuum degree all have important effects on the quality of the plate.

Development: Taking advantage of the water solubility of photosensitive paste, the unexposed photosensitive paste is washed away with water. The development method has a great impact on fine screen printing. Before development, water must be sprayed to make the photosensitive paste absorb water and swell. Let it stand for 1 "2 minutes, then use a high-pressure water gun to develop it back and forth in a fan shape until the image is completely clear.

Note: High pressure water washing should not be too close to the screen, usually 0.8 "1m, otherwise the pressure will be too high and the lines will easily produce jagged edges, and in severe cases, some of the dots will be washed away

Drying: Dry the moisture on the screen, and the temperature should not be too high, otherwise it will cause changes in the tension of the screen, usually at 40 "and 45 ℃.

Revision and inspection: Repair and inspect pinholes and some NPTH holes.

Final exposure: further improve the adhesion between the photosensitive paste and the mesh, and increase the service life.

Sealing: Fill the empty part of the screen with sealing paste to prevent ink leakage during printing.

⑵ Direct PCB board making method for photosensitive film

Process flow: Stretching the mesh - degreasing - wetting - film application - drying - reinforcement - exposure - development - drying - repairing - sealing the mesh

method

Photographic film, commonly known as water film, is made by using a transparent plastic film with a thickness of 0.1mm as the base and coating one side with a certain thickness of photosensitive emulsion. When using, the screen is fully wetted, and then the film is attached. The film is adsorbed onto the screen by capillary action, dried, and the plastic base is peeled off for exposure and development, and finally the desired pattern is obtained.

Wetting: Using wetting to promote the formation of a uniform water film on the surface of the screen, in order to facilitate the smooth transfer of the photosensitive film.

Film application: Apply the photosensitive film onto the wet surface of the screen scraper according to the type of screen, and then use a scraper to remove excess water after application.

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

The other operations are consistent with the photosensitive paste plate making method.

2. Indirect PCB manufacturing method

method

The method of indirect plate making is to first expose the indirect film, harden it with 1.2% H2O2, develop it with warm water, and dry it to make a peelable graphic negative film. When making the plate, the adhesive film surface of the graphic negative film is tightly attached to the stretched screen, and the adhesive film is pressed tightly to the wet screen by squeezing. After removing the film substrate, the screen printing screen is made by blowing it dry with wind.

Process flow:

1. Web stretched - degreased - dried

2. Indirect film - exposure - hardening - development

1and2- bonding - drying - repairing - sealing

3. Direct indirect plate making method

method

The method of direct indirect plate making is to first place the photosensitive material coated wrist base photosensitive film face up flat on the workbench during plate making, and then place the stretched wrist mesh frame flat on the film base. Then, place the photosensitive paste in the mesh frame and apply pressure with a soft scraper. After drying thoroughly, remove the plastic film base and attach the photosensitive film wrist mesh to it, which can be used for plate making. After development and drying, the silk screen is produced.

Process flow:

Stretching the net - degreasing - drying - peeling off the substrate - exposure - developing - drying - repairing - sealing the net