Part 5 - Etch inner layers
Channel: Eurocircuits
Date: 08/05/2014

We remove the unwanted copper using a powerful alkaline solution to dissolve (or etch away) the exposed copper from the inner layer.  The process is carefully controlled to ensure that the finished conductor widths are exactly as designed.  But designers should be aware that thicker copper foils need wider spaces between the tracks.  The operator checks carefully that all the unwanted copper has been etched away.

http://www.eurocircuits.com/index.php/making-a-pcb-eductional-movies/38-etch-inner-layers