The Seven Year ETCH - by Matt Stevenson
Once a circuit board has had all of the layers laminated together, through holes drilled in it, been copper plated (both electroless and electroplated) and the outer layer images applied, it is time to physically create all of the traces and pads. In order to do that we need to etch away all of the copper between these desired features. Remember that we use a fully copper clad panel to create our PCB, then we apply the images (cover the unwanted areas with photoresist) and electrodeposit approximately 1-1.2 mils of copper to the exposed traces, pads and through holes. Once that is complete we plate about 0.3 mils of tin on top of that copper. The tin is applied to protect the desired copper during our etching process. The photoresist is then chemically removed from the surface of the panel exposing the unwanted copper below it.
For etching of outer layer copper, the most common etchant used is an ammoniacal (ammonium chloride). The chemistry behind its reaction to copper in the etching process is quite complex, I won’t go into the details but if you are interested there are several good write-ups posted online to satisfy your chemical cravings. The equipment is equally complex, full of spray chambers, pressure control, speed control, rinsing, replenishing and monitoring pH and a specific gravity of the solution.
The process of etching a PCB, especially a dense or impedance controlled design is more of an art form. Though there are starting point settings for most every combination of copper thickness and line width, the final tweaks needed to ensure that each job is etched properly requires an understanding of all of these variables and how they interact during the process. On a complex PCB design, typically a first article etch panel is processed under the current process. Critical traces and impedance measurements are taken and the etcher is fine-tuned to get the line widths and impedance centered in its range.
Once the panel is etched the tin protecting the desired copper is chemically removed. The result of this process is generally a fully electrically functional PCB. Other steps need to be taken to protect the copper at assembly and oxidation but all of the nets should be defined physically at this point.
Design Tips: As with the copper plating portion of manufacturing, some design decisions can have an impact on the manufacturability of the PCB at the etch process as well. Etching is really an exercise in getting fresh etchant chemistry in contact with the copper and sweeping away the byproducts as quickly as possible. Isolating critical traces on your design without much in terms of copper around it can lead to over-etching of these traces (or etching more than the traces that are surrounded by copper). On the flip side, critical traces embedded in copper pour with limited space can have the opposite problem. Replenishing the etchant solution becomes more challenging and these traces can exhibit mild under etching. Adding copper thieving around isolated traces is helpful to maintain the intent of the trace and providing a bit more space between the trace and the embedded plane (8-10 mils for half oz base copper) is a great way to keep all of the features etched close to their nominal values.