Dry film is a polymeric compound which, when irradiated by UV light, can produce a polymerisation reaction (the process of synthesising a polymer from a monomer) to form a stable substance that adheres to the surface of the board, thus blocking the function of plating and etching.
Dry films can be divided into four categories depending on thickness: (0.8 mil, 1.2 mil, 1.5 mil, 2.0 mil)
0.8 mil thickness dry film is mainly used for FPC fine line production.
1.2mil dry film is mainly used for inner laminate work.
1.5mil dry film is mainly used for outer board work, but it can also be used for inner board work, but because of the thicker etching process, it is easy to cause side etching and relatively high cost, so it is generally not used for the inner layer.
2.0 mil dry film is mainly used for some special requirements of the board, such as the larger secondary holes 1.5 mil dry film can not meet the requirements, only used to.
Main characteristics of dry film.
Under certain temperature and pressure, it will adhere firmly to the board surface.
When exposed to a certain amount of light energy, it absorbs energy and undergoes a cross-linking reaction.
The part that is not irradiated by light is not cross-linked and can be dissolved by weak alkaline solutions.
Dry film storage environment.
Constant temperature, constant humidity, yellow light safe area. Prevent storage with chemicals and radioactive materials.
Other common quality defects in dry film include: residual glue development, exposure to foreign bodies, poor vacuuming, poor development, excessive development, open circuits, short circuits, line bumps, gaps, exposed copper, dropped film, line jaggedness (dog teeth), etc.
Common problems in the dry film process
1. Residual film left on the copper surface after development (unclean development).
2. Poor effect of hole masking method.
3. Thin wires or photoresist layer in unexposed areas not easily washed out during development (poor exposure).
4. Damage to the dry film during development or uneven edges of the dry film after development (overdevelopment).
5. Tin bleeding due to warped edges of the photoresist film when the wire is plated with tin-lead (bleeding plating).
6. Wrinkling of the dry film.
7. Air bubbles between the dry film and the copper surface of the substrate.
