PCB Stripping Process
Post etching process, the copper present on the PCB gets covered either by electroplated tin or tin/lead. You can use concentrated Nitric acid to remove the tin. Nitric acid is very effective in removing tin, and does not damage the copper circuit tracks below the tin metal. Thus, now you have a clear distinct outline of copper on the PCB. The board can now be passed on to the next process.
This is one of the most important processes in designing of a PCB. The areas on the circuit board that cannot be soldered are covered with a solder resist material. By doing this, it avoids the solder from forming traces, which can create shorts to nearby component leads.