When it comes to reworking, parylene's strengths are also its biggest drawbacks. In addition to its ability to comprehensively coat substrates, it is also, by design, very challenging to remove. However, "very challenging" and impossible are two different things. Furthermore, good planning strategy before coating can also help to reduce the need for parylene rework.
The Parylene Challenge
Parylene dimer has to be vaporized to coat the item. Because it deposits as a vapor, it is extremely good at creating a truly conformal coat. This is the first of many challenges in reworking it. It isn't easy to simply peel away.
In addition, parylene offers multiple types of resistance to reworking:
- It is relatively strong and hard, making it very challenging to chip away or otherwise mechanically compromise.
- Even though it is not a particularly sticky substance, its ability to permeate even the smallest crevices on the coated item typically give it strong mechanical adhesion even in the absence of chemical adhesion.
- Parylene is extremely chemically inert. Moisture, salt, corrosive compounds and most solvents have little to no effect on it. It is also neither affected by acids nor bases.
- While heat can break parylene down, since it is stable to temperatures as high as 350 degrees Celsius (or higher in a vacuum), melting it away might also end up melting the items that it coats.
Parylene can be reworked, but it isn't easy. Currently, there are three ways to remove and rework parylene, with additional methods under development.
Mechanical removal can be challenging and time consuming, but parylene is not impervious to the effects of hard, physical work. It can be cut, sanded or scraped way. Furthermore, given than parylene coatings are typically very thin, using this approach might not be as time- or labor-intensive as it first seems. However, mechanical means carry the risk of damaging the coated item if the person doing the reworking goes too deep into the item.
Microabrasion is also a mechanical method of reworking, but it generally leads to a better end result. When parylene is reworked through microabrasion, the operator uses a stylus that uses compressed air to deliver fine abrasive particles. As they strike the parylene and gradually break it down, a vacuum sucks up the particles and the removed parylene. This is a more gradual process than using more blunt mechanical means, but usually leads to a much lower risk of damage.
Finally, parylene can be melted away. Thermal removals are challenging because parylene has better heat resistance than many of the plastics it coats. Additionally, even if the coated item is adequately heat resistant, the thermal removal method could leave discoloration behind. Nevertheless, given the right materials, it can be a suitable alternative to mechanical and microabrasive methods. Additionally, some rework processes use heat to soften the parylene to make mechanical removal easier.
Researchers have recently found ways to use specialized lasers and jets of plasma to not only remove the parylene from the coated item but also to eliminate any debris that would otherwise be present. Thanks to these researchers, parylene removal via laser ablation is available from a limited number of highly advanced vendors.
The best way to rework parylene is to avoid having to rework it in the first place. One way to avoid reworking is to mask the item to be coated before it is placed in the deposition chamber. That way, the parylene deposits where it needs to, but leaves other areas uncoated, saving you from having to rework it.
Another major cause of parylene reworking is when the coating does not get applied properly the first time. Typically, there are two ways to avoid this problem. First, the item to be coated must be perfectly clean, preventing debris from being coated to the item. Second, because parylene does not always adhere well to smooth metal surfaces, those areas should be coated with a chemical like A-174 silane that serves as an interface between the parylene and the surface, increasing adhesion.
Finally, designing your products from the start with parylene coating in mind can also reduce the need to rework it. One strategy is to completely seal around areas that do not need to be coated, removing the need to mask prior to deposition or reword afterwards. Using metals and other materials that allow for better parylene adhesion can also reduce the need for reworking after the fact. Involving your conformal coating specialists early on in the design of your product can help to ensure better outcomes.
Ultimately, parylene can be reworked. However, it is almost always best to avoid rework if at all possible.