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As many of you probably already know, PCB etching seems like a simple task on the surface — selective removal of material using an etching agent. There are quite a few things that can go wrong during this process. Over-etching, copper areas that detach from the board, issues with etching solution and even accidental removal of critical components. Considered by many to be somewhat of an art form, PCB manufacturers work hard to create a better etching process (Remember acid traps?). Adhering to best practice and continuous improvement is a must, to help us to avoid issues with your finished board.
This is how we do it.
Once all of the layers are laminated together, through holes drilled, copper plated (both electroless and electroplated) and the outer layer images applied, it is time to physically create all of the traces and pads for a circuit board via the etching process.
Etching defines the distinctive routes of a PCB board. It is also the process that tests the quality of a design by answering the design questions that will determine its functionality. Did you leave enough space? Are your traces wide enough?
PCB manufacture etching is a reductive process, not an additive process. The board starts with a consistent layer of copper throughout and the etching process removes all unwanted copper. Now, if you think that sounds wasteful, don’t worry; the copper removed during etching is easily recaptured and recycled.
Remember that we start the PCB process with a fully copper-clad panel. After coating it in a light sensitive polymer, we project high-intensity UV through an image of the PCB design that you so carefully composed. A photo resist layer covers the areas where we want to remove the copper. It is a photo negative of the traces you want to keep.
Before etching, copper (approximately 1-1.2 mils) is plated on exposed traces, pads, and through holes. We plate about 0.3 mils of tin on top of that copper, which will protect the wanted copper from the etching chemical.
The photoresist is then chemically removed from the surface of the panel to expose the unwanted copper and it is time for the etch.
There are a few different chemicals used for etching copper, but the most common is an ammoniacal (ammonium chloride). The chemistry behind its reaction to copper in the etching process is quite complex. We won’t go into the details here, but if you are interested there are several good write-ups online to satisfy your chemistry cravings.
The equipment we use for the etching is equally complex. The board passes through carefully controlled spray chambers at a carefully controlled speed. The ammonium chloride dissolves all the copper not protected by the tin we plated on the PCB design pattern. Getting it right requires precise conveyor speed, pressure, pH, and specific gravity.
Even with all this precision, etching is really more of an art form; especially when a design is dense or impedance-controlled. 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 understanding of all of these variables and how they interact during the process.
On a complex PCB design, the initial etch panel is processed for first article inspection using a standard process. Critical traces and impedance measurements are then taken, and the etcher is fine tuned to get the line widths and impedance centered in its range.
Once the panel is etched, we use a chemical process to remove the tin protecting the desired copper. 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 prevent oxidation, but all of the nets should be defined and operational at this point. We have etched our way to a functional electrical part!
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.
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 copper around it can lead to over-etching (that is, etching more than the traces that are surrounded by copper) of these traces.
Critical traces embedded in copper pour with limited space can have the opposite problem. Replenishing the etchant solution becomes more challenging. These traces can exhibit mild under-etching.
For every half ounce of copper, a good design has 8-10 mils of space between the trace and the embedded plane. Adding copper thieving around isolated traces helps to maintain the intent of the trace and provides a bit more space, keeping all of the features etched close to their nominal values.
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