What is Circuit Design
Circuit Designers try to make connections.
3. Process Overview
Etching removes copper from a PCB. Heavy copper must spend more time in the etching tanks
In the etching process, both sides of the PCB are laminated with a special plastic material that polymerizes when exposed to light. Digital projectors shine light in a pattern that is unique to each copper layer from the design file. This light causes the plastic to toughen. Parts of the plastic that were not exposed to light will wash away in a chemical bath.
The exposed copper dissolves from the board in an acid bath.
Many engineers do not realize this, but the current state of manufacturing does not allow the copper to etch vertically down -- it undercuts at a slight angle. So to ensure a minimum trace width where the copper meets the dielectric, fabricators have to make all of your copper traces a bit bigger than you asked for. This is referred to as etchback compensation and it is the reason for the minimum spaces in trace and space charts. There has to be enough room between the traces after etching compensation to allow the acid to get in and dissolve the copper.
This chart of preferred minimums at Royal Circuit Solutions. These minimums can be exceeded at the expense of decreasing yields.
You'll notice that the minimum space for outer and inner traces differ by several mils. Manufacturers can exceed these minimums if asked, but it will increase the cost of manufacture. Going beyond these limits is not impossible, it is just a bit risky as it can reduce overall yields.
 Some production methods allow the etching of copper with high-powered lasers that vaporize the copper layers. Most production lines still rely on acid-baths to remove the copper.