Photosensitive plates can be negative and positive. Currently, negative plates are not used since today the quality obtained from a positive emulsion is quite good and its elaboration is more direct than if we used negative emulsions that would then force us to invert the drawing using an inverter paper or another technique. Our Positive Fiberglass Circuit Boards are fiberglass substrate plates also known as FR4, which have a thickness of 1.6mm and 35 µm of copper on one or both sides of it. As a material used, we supply plates from Bungard, which is a leading German manufacturer of this type of plates.
The process for obtaining a printed circuit is not very complex, but requires exposure or insolation times that, depending on the type of light used, the absorption of light by the photolith, and its quality may vary.
The manufacturer of this type of plate gives us an orientation of the insolation times using a professional insolator, and a professional photolith, but if that is not the case, we ourselves will obtain our times based on our experience and based on testing. failures.
After the insolation, we proceed to the development, which is the moment that our printed circuit appears clearly painted on our board. When the development process is finished we have to eliminate the excess copper that is the unpainted part. For this we introduce our plate in what we call acid.
In the past, either ferric chloride or a fast etchant was used. Nowadays, at least nationally, ferric chloride is no longer used and only the fast attacker is used. They are two liquids that when they come together, the oxidation-reduction reaction (redox) begins. An electron transfer reaction occurs as occurs in batteries and cells. The species that loses electrons is oxidized and the species that gains them is reduced. In this way, when introducing our plate into this mixture in action, the copper not covered with the emulsion or drawing oxidizes and passes into the liquid that forms the mixture, leaving only the drawing of our printed circuit on the substrate.
There are different combinations of liquids to attack copper. Hydrochloric acid + Hydrogen peroxide, Hydrochloric acid + Sodium perborate, ...
Let's say it's not difficult, but it requires a little learning.