In the previous step, the copper surface is covered with a layer of tin as an etching resist. Now we need to remove the unwanted copper from the surface to obtain the final desired circuits. This process consists of three main steps.
Film Stripping
After pattern plating, the copper on the unplated parts of the substrate is covered by dry films. These films need to be etched away to expose the copper surface when the circuit patterns are finally formed. The film stripping solution is the dilute alkali, which dissolves the resin containing acidic group in the dry films through a neutralization reaction. Finally, the dry films are stripped off, thus exposing the copper surface to be etched.
Etching
After the dry films are removed, the board is ready for etching. The exposed copper surface is removed by the alkaline etchant, while the copper surface protected by tin will be retained as the conductive pattern. During etching, the etching method, types of etchants, the pH value of the etchant, the etching rate, or other factors can affect the quality of circuits. Therefore, our engineers will sample the solution used in this process, or other processes that require solutions, and send them to the chemistry laboratory for standard quality testing to ensure the correct reagent ratio and the quality of the circuits.
Tin Stripping
After etching, we need to remove the thin layer of tin on the top that protects the circuits. In this process, the tin stripping solution (mainly nitric acid) is used to dissolve the tin layer to expose the copper surface. After completing these steps, the basic PCB circuits are formed. The board is then transferred to the automatic optical inspection area to check the etching quality.
