Before getting to know phototool, the manufacture process of printed circuit board shall be introduced.
In the conventional manufacture of printed circuit boards (PCBs), a laminate comprising a dielectric substrate having a copper sheet affixed to at least one side is prepared. The copper surface is overlaid with a photo-resist layer which is typically a carboxylic acid containing polyacrylate.
A phototool is prepared which is a negative image of the desired copper electrically conductive circuitry, and this is typically a silver halide photographic emulsion, or diazo phototool film with emulsion.
The phototool is placed over the photo-resist layer and irradiated with actinic radiation, such as ultraviolet (UV) light. This causes the photo-resist layer which is exposed to the actinic radiation to polymerize and harden, thus producing a latent negative image of the desired circuitry in the photo-resist layer. The unexposed areas of the photo-resist layer which have not been exposed to actinic radiation are then removed using mildly aqueous alkali to expose the copper surface, and this is then removed by chemical etching, thus resulting in a dielectric substrate containing the required copper circuitry covered by polymerized photo-resist. This photo-resist is finally removed to yield a dielectric substrate having the required copper electrically conductive circuitry.
There are two different types of phototool made of different material, while serve the same purpose in PCB manufacturing, silver halide film, and diazo film, the name of it define what it made of.
Silver halide phototool also known as silver master film (black film), major manufacturers of it includes famous film company Fuji film, Agfa, etc.
Diazo phototool is of more competitive cost and durability, generally can be used for over 300 times of exposure. Diazo Film designed to function as a working master in the manufacture of printed circuit boards. Unlike silver halide films, diazo can be handled in yellow room light and is processed using ammonia vapors and heat.