How Easy is Parylene to Rework?

When it comes to reworking, Parylene’s strengths are also its biggest drawbacks. In addition to its ability to conformally coat substrates, it is also, by design, very challenging to remove. However, a good planning strategy before coating can help to reduce the need for Parylene rework.

The Parylene Challenge

Parylene dimer is deposited as a vapor. Therefore, it is extremely successful at conformally coating applications as a long-lasting protective solution. Parylene’s application and unique protective properties  make it difficult to remove and rework:

1. It is relatively strong and hard, making it very challenging to chip away or otherwise mechanically compromise.
2. 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.
3. Parylene is 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.
4. While heat can break Parylene down, it is stable to temperatures as high as 350°C (or higher in a vacuum). Melting the coating away might also melt and damage the coated object in the process.

Reworking Parylene

Parylene can be reworked, but it is difficult. 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 that Parylene coatings are ultra-thin, using this approach might not be as time or labor intensive as it first appears. However, mechanical removal risks damaging the coated item if the person completing the reworking cuts 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 utilizes a stylus that expels fine abrasive particles using compressed air. As the abrasive particles strike the Parylene and gradually break it down, a vacuum sucks up the particles and the removed Parylene. This gradual process is less likely to damage the coated item than blunt force mechanical removal.

Finally, Parylene can be melted away. Thermal removals are challenging because Parylene is typically more thermal resistant than the coated object. 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 micro-abrasive methods. Additionally, some rework processes initially use heat to soften the Parylene to make mechanical removal easier. Researchers have 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.

Strategic Alternatives

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 before it is placed in the deposition chamber. Masking allows the Parylene to be deposited where it needs to, but leaves other areas uncoated, which saves you from having to rework it.

Another major cause of Parylene reworking occurs 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, which prevents debris from being coated to the item. Second, because Parylene does not always adhere well to smooth metal surfaces, metal areas should be coated with a chemical like Silane A-174 adhesion promoter that serves as an interface between the Parylene and the surface.

Finally, introducing Parylene early in the design phase can also reduce the need to rework it. One design strategy is to completely seal around areas that do not need to be coated, removing the need to mask prior to deposition. Using metals and other materials that allow for better Parylene adhesion can also reduce the need for reworking. Involving conformal coating specialists early on in the design of your product can help to ensure better outcomes.