Parylene's benefits as a conformal coating are well known. It resists heat, cold, moisture, and pressure; salt spray, electricity, and solvents can't permeate it. And while these attributes of parylene contribute to the conformal coating's appeal, they also present distinct challenges, particularly in regards to parylene removal, rework, and repair.
Because of parylene's strength, for example, many of the methods used to remove or repair other conformal coatings won't work. You can't simply submerge parylene in a solvent like you might with an acrylic-coated component, for instance. Parylene isn't completely impervious to removal tactics, though. Focused heat, mechanical, and microabrasion methods can all be effective means of parylene removal.
Although parylene is heat-resistant, it can't always take the heat, so to speak. Localized application of extreme heat, such as with a soldering iron, can, in fact, melt or burn through a parylene conformal coating. However, this parylene removal method is often a poor or risky choice for many applications; the associated extreme heat and prolonged exposure can damage PCBs and temperature-sensitive substrates, for example. After all, parylene has a higher melting point than many plastics.
Parylene removal using thermal means can also cause discoloration and yield residues. Furthermore, it is important to perform thermal parylene removal in a well-ventilated area. For these reasons, thermal methods are often regarded as the least-recommended means of parylene removal.
Parylene also isn't impervious to a dedicated physical attack, and a little elbow grease can go a long way. Mechanical parylene-removal methods such as scraping, sanding, picking, and cutting can be effective. The drawback to mechanical parylene-removal approaches, however, is that parylene conformal coatings can be very thin—sometimes as little as 0.2 mils. Although usually an advantage, parylene's ability to be applied in extremely thin layers renders the coating vulnerable to inadvertent removal of too much material that can consequently damage the underlying substrate or components.
For most applications, the best parylene removal method is microabrasion. Related to the mechanical process, micro-abrasion is much more controlled, leading to better results. These systems use a stylus with a tiny nozzle to direct a stream of pressurized air and abrasive media at the parylene-coated component. The system can be handheld or automated for finer control. As the abradant gradually wears down the parylene coating, a vacuum captures both the abradant and the removed parylene. This method provides markedly better results than mechanical and thermal parylene-removal methods with less potential for damage.
Alternative Removal Methods
With none of the aforementioned parylene-removal methods providing a perfect solution, researchers have explored additional approaches. Researchers in Germany, for example, have used 193-nm excimer lasers and plasma jets together to not only remove parylene but also to remove the debris generated by the removal process. The combination of the two technologies reduces the need for an additional cleaning process after the removal as well. Another patented process uses a solvent called tetrahydrofuran to soften the parylene coating. While the tetrahydrofuran can't remove the parylene, it does make it easier to remove through mechanical means, although it does carry the risk of spilling over and compromising the coating on other areas of the component.
Avoiding Parylene Rework
Because parylene-removal is such a challenging process, the best plan of action is to avoid having to do it in the first place. Properly preparing the substrate prior to coating yields better adhesion; preparation includes both cleaning the substrate and, if necessary, employing an adhesion promoter such as A-174 silane. In addition to making sure that the parylene coating is applied where required, it is equally as important to prevent "no-coating zones" from being coated. Properly masking your substrate before placing it in the deposition chamber can also help to reduce or eliminate rework.