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20 DESIGN007 MAGAZINE I MARCH 2020 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 (Figure 2). We plate about 0.3 mils of tin on top of that copper, which will protect the want- ed 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 am- moniacal (ammonium chlo- ride). 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 sat- isfy your chemistry cravings. The equipment we use for the etching is equally complex. The board passes through carefully controlled spray chambers at a care- fully controlled speed. The ammonium chlo- ride dissolves all the copper not protected by the tin we plated on the PCB design pat- tern. Getting it right requires precise convey- or speed, pressure, pH, and specific gravity (Figure 3). Even with all this precision, etching is more of an art form, especially when a de- sign is dense or impedance-controlled. Though there are starting point settings for almost ev- ery combination of copper thickness and line width, the final tweaks needed to ensure that each job is etched properly requires an under- standing 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 chemi- cal process to remove the tin protecting the de- sired copper. The result of this process is gen- erally a fully electrically functional PCB. Other Figure 2: Before etching. Figure 3: Etch machine.

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