Three PCB Fabrication Styles & Their Process Flows

Description

PCB fabrication styles are divided into direct fabrication system, direct - circular fabrication system, and circular fabrication system. The accoutrements used are photosensitive paste, photosensitive film, and circular film independently.

Direct Fabrication system

Method

Apply a certain consistence of photosensitive paste (generally diazo swab photosensitive paste) on the stretched screen. After coating, dry it, then attach the plate - making negative film to it and place it in a plate - making machine for exposure. After development, irrigating, and drying, it becomes a screen printing screen.

Process Flow

Preparation of photosensitive paste - Stretched screen - Degreasing - Drying - Film coating - Drying - Exposure - Development - Drying - Plate form - Final exposure - Screen sealing

Styles and Functions of Each Section

• Degreasing: Use a degreaser to remove the grease on the screen so that the photosensitive paste and the screen can be completely clicked together, and the film isn't easy to peel out.

• Drying: Sot the humidity. To avoid the change of pressure caused by too high temperature of the screen cloth, the temperature should be controlled at 40 - 45 °C.

• Preparation of Photosensitive Paste: Mix the photosensitizer unevenly with pure water, add it to the photosensitive paste, stir well, and let it stand for 8 hours before use.

• Film Coating: Use a squeegee to unevenly apply the photosensitive paste on the screen. According to the coating system, it can be divided into automatic fleece coating and homemade coating. The number of coating times can be determined according to the factual situation. When coating, first fleece the squeegee side to fill the gaps between the screen yarns to avoid air bubbles. Then cover the printing side (the side in contact with the PCB). Presently, each coating by the automatic fleece can increase the film consistence by about 3μm. Thus, the coating system for the solder mask screen is substantially named as follows: Coat the squeegee side doubly - Dry - cover the printing side three times - Sot - cover the printing side three times - Sot - cover the printing side three times - Sot.

Description of Incorrect Film Coating

A. The consistence of the carpeted squeegee side and printing side is applicable and meets the conditions.

• Disadvantage of thin coating (printing side): Poor continuity.
• Disadvantage of too thick coating on the squeegee side: Because the photosensitive paste on the squeegee side is too thick, the photosensitivity is uneven. After washing with water during development, the face is rough, and the essay penetrates into the film subcaste, causing the film subcaste to peel out and syncopating the screen life.
• Disadvantage of too thin coating on the squeegee side: Poor continuity.

Drying

Sot the photosensitive paste unevenly to avoid the situation where the outside of the photosensitive paste is dry while the inside is wet. If the temperature is too high, the outside of the photosensitive paste will dry first while the inside is still wet, which will dock the screen life. The temperature should be maintained at 40 - 45 °C, and the time is about 10 twinkles. Acclimate the drying time meetly according to different film density.

Exposure

Applicable exposure can make the photosensitive paste polymerize under light, and a clear image can be developed through the negative film.

Factors Affecting Screen Quality

• Correct exposure energy.

• Vacuum degree during exposure.

• Cleanliness of the glass of the exposure machine.

• Generally, the exposure energy is acclimated by the exposure time. During product, the canted exposure system should be espoused according to the screen mesh count and film consistence, and the correct exposure time of colorful defenses should be measured by using the exposure test piece.

Operation system of the Test Piece

• ① Expose for a time that's twice the pre - estimated exposure time, develop in the normal way. After development, elect the one with the stylish effect, that is, the clearest image range. Then multiply the factual exposure time by the measure marked on the named image to get the better exposure time. There are 5 portions on the test piece, videlicet 1.0, 0.7, 0.5, 0.33, and 0.25. Each measure corresponds to a indirect target pattern and blotches.

• ② If the measure 1.0 seems to be the stylish measure, double the former exposure time and re - make the plate for exposure testing.

• ③ If the measure 0.25 seems to be the stylish measure, halve the former exposure time and re - make the plate for exposure testing.

• ④ If several successive portions are satisfactory, when printing fleck patterns, use the measure with a lower upper limit, that is, a shorter exposure time; when publishing fairly thick lines, use the measure with a advanced lower limit, that is, a longer exposure time.

• ⑤ If several successive portions are satisfactory, elect the stylish measure according to the clarity of the thinnest lines or blotches of the indirect target, depending on the type of the screen being made.

  In addition, the close contact of the negative film, the cleanliness of the glass of the exposure machine, and the vacuum degree all have important impacts on the plate - making quality.

Development:

Use the water - answerable characteristics of the photosensitive paste, and wash down the unexposed photosensitive paste with water. The development system has a great impact on the fine screen. Before development, spot water to make the photosensitive paste absorb water and swell first, let it stand for 1 - 2 twinkles, and then use a high - pressure water gun to develop back and forth in a addict - shaped manner until the image is fully clear. Note: The high - pressure water marshland shouldn't be too close to the screen, generally 0.8 - 1m. Else, the pressure is too high, which is likely to beget serrations on the lines, and in severe cases, some blotches will be washed down.