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For high reliability electronics in many industries including Aerospace, Defence and Medical, Parylene coatings tend to be the first choice for environmental protection. However being highly resistant to chemicals, gases, liquids, temperature and electrical exposure does make removal and repair a challenge.
While Parylene is heat-resistant, ‘C’ the most common variant for electronic applications will burn off where high temperature is concentrated. A soldering iron under a vacuum hood could be used to burn off at the solder points. Where it is raised from the board the component then lifts off. If the component is sitting tight to the board a blade could be used to carefully cut around the component to make removal easier.
This might be effective for small repairs though it is not the cleanest method, residues and discolouration may result. As Parylene has a high melting point this approach could also pose a risk to heat sensitive substrates.
Laser ablation removes material from the surface by irradiating it with a laser beam. The beam can drill, cut or mark delicate materials with great precision.
Laser ablation, removes very precise areas and tends to fit high density difficult to mask parts. Applications such as microelectronics, electro surgical devices and implantable electrodes that require very small, precise and intricate shapes to be Parylene free would be suited to this approach. It is less likely to be used for the purpose of general rework as it requires capital investment and specific programming that would not be viable for ad-hoc repair and rework.
Parylene is highly chemical resistant. No chemical can dissolve it are room temperature. However Tetrahydrofuran an organic solvent can soften the coating; temporarily loosening the adhesion bond between the coating and the substrate so that it can be lifted off like a mask with a tweezers. It is not suited to working with specific areas; in the case of rework and repair this is usually desired.
Parylene is a soft polymer and as such physical, mechanical removal is probably the best method to remove the coating. Using coarse methods such as picking, scraping and cutting can be effective but risk damaging delicate track lines and components.
The most common method to remove Parylene is micro abrasion. This is the fastest and easiest method for both spot and whole board removal. Desktop equipment can be used with a hand held stylus directing pressurized air and abrasive media. Wheat starch is the best for ESD sensitive boards; other common media include sodium bicarbonate and plastic beads. For finer control automated systems are also available.
Once the section is repaired the specific area could be covered with other protective material however for optimum protection the whole board may be recoated in Parylene.