The photoresist etching process depends on the correct chemical reactions.
PCB fabricators configure equipment to ensure quality within the photoresist etching process.
Photoresist etching for PCBs must meet stringent specifications.
One of the most problematic aspects of building a realistic scale model of an actual spacecraft is scale. In this instance, I’m not referring to a scaly reptilian creature, the bathroom scale that lies about my weight, or the scale that can accumulate on water pipes. Instead, I’m referring to achieving the correct dimensions when attempting to build a small scale model of a huge spacecraft.
I love building models, but the injection molding methods used by manufacturers cannot reproduce the solar panels for a Soyuz spacecraft to an accurate scale. If we had the technology to expand my 1/144 scale model to full scale, we would see six-foot-thick solar panels that would weigh enough to pull the Russian transport out of its correct orbit.
However, an aftermarket solution exists. Various suppliers produce photo-etched replacements that can add wispy-looking antennas, straps, instruments, and, of course, solar panels to any spacecraft. The photo-etched parts consist of precisely manufactured, laser-cut thin metal pieces. With the use of the right tools and photoresist etching, a modeler can build a spacecraft suitable for any miniature astronaut or cosmonaut.
Photoresist Etching on PCBs
Large-scale PCB manufacturers use electroplating and etching processes to produce the traces on a board. With electroplating, the production process begins with electroplated copper covering the substrate of an outer layer board.
Photoresist etching also functions as another key step for producing printed circuit boards. Protecting the desired copper during the etching process requires a balance between removing undesired copper and leaving the resist in place. The protection occurs through the application of a thin coat of etch resist—consisting of a mostly tin mixture—to the circuit pattern, protecting the desired pattern from the etchant.
Because etching removes any excess copper from a blank, clean board, the thickness of the copper plus the thickness of the plating cannot exceed the thickness of the photoresist. The subtractive process of first removing the excess copper, then the resist, produces the circuit pattern. Although fabricators can use buckets, tanks, or spray machines to apply the etchant, most opt for high-pressure spray equipment that can etch a standard-sized PCB in less than one minute.
While the etchant or stripper is usually classified as an ammoniacal etchant, the general ingredients typically include ammonia/ammonium chloride or an ammonia/ammonium sulfate for spray etching. The resist prevents the etching solution from contacting the desired conductive patterns. Because the etchant does not affect the resist, it removes only the undesired copper.
During the etching process, the reaction of the ammoniacal etchant with copper produces large quantities of cuprous ions from the cupric ions of the copper. The excess amount of cuprous ions cause a non-bright, unlevel coating to form on the plating and harm conductivity. To counter the overload of cuprous ions, etching equipment pulls air into the etching chamber. Oxygen absorbed from the introduced airflow causes re-oxidation of the cuprous ions into cupric ions.
Steps to Removing the Resist
Stripping away the resist requires a process that involves oxidation and reduction, or an oxidation-reduction reaction. While oxidation with nitric acid removes four electrons from the resist, the process tarnishes the copper and produces copper oxide. Reduction reduces the amount of the oxidizing agent by reducing the amount of oxygen that adds two electrons to the copper.
Oxidation to remove the resist and reduction to protect the copper starts the process. Removing the tin requires complete oxidation and then requires dissolving the tin into a solution through the application of stannic salts. Unfortunately, copper has a much softer consistency than tin and, as a result, can strip away before the tin. Manufacturers use inhibitors in the etchant to prevent the copper oxidation from harming the conductive surface of the PCB.
Photoresist Etching Requires Attention to Quality
A completed, functional PCB cannot have problems with etch quality. Otherwise, the PCB will not meet the specifications required for use in consumer and industrial products. Fabricators define etch quality in terms of the uniformity of the trace edges, and the amount of etch undercut. Because the etchant can flow in any direction—including sideways and downwards—the etchant undercuts the trace.
Fabricators apply a formula called the “etch factor” when considering the quality of the board. The “etch factor” equals the amount of undercut divided by the amount of etched copper. Minimizing the amount of undercut occurs through the configuration of the production equipment and adjusting the etch chemistry through the use of banking agents.
Along with preventing edge undercut, fabricators also work to protect the production process from residual photoresist. Any unstripped photoresist can leave a copper foot next to a trace that decreases the distance between traces. In addition to preventing the buildup of residual photoresist, fabricators also attempt to minimize any puddling of etchant on the board surface. Etchant puddling can allow different etch patterns to form on the PCB.
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