Removal of Conformal Coatings
Conformal coatings provide reliable protection for substrate surfaces of printed circuit boards (PCBs) and related electrical components. However, these coatings sometimes require removal for repair or other purposes. Many coating materials can be removed by abrasive, mechanical, plasma and thermal techniques, but chemical methods are typically the most popular for removing acrylic, epoxy, silicon, or urethane conformal coatings. Chemical removal does the least damage to PCB components.
However, no single chemical material/process is equally successful for all uses. In some cases, no chemical removal solution is applicable. One might think chemical substances are suitable for removal of superior parylene coatings, since its substrate-application employs a unique chemical vapor deposition (CVD) process. However, this is not so. Chemical removal of parylene is not recommended in the vast majority of cases; other techniques remove the parylene more reliably, with less damage to underlying components.
Are Any Chemical Removal Solutions Available for Parylene?
CVD-applied parylene coatings generate excellent conformal resistance to abrasion, chemicals, humidity, moisture, and extreme changes in temperature. However, chemical inertness is a basic property of parylene, complicating its chemical removal. Under these circumstances, the liquid stripping techniques customarily used for most other conformal coatings are generally ineffective for parylene, a consequence of its exceptional chemical resistance and otherwise inert nature.
Tetrahydrofuran (THF) is a colorless organic compound whose chemical formula is (CH2)40. It demonstrates:
Because of these factors, THF has been successfully used for removal of parylene coatings from substrates. If, for instance, the parylene coating has been applied at a thickness of .001 mm, the coated assembly can be immersed in the THF-based solvent for approximately two to four hours to affect removal. During immersion, the parylene coating begins to separate from the assembly's surface. Physical removal of the parylene with tweezers is enacted after the assembly has been rinsed in alcohol and allowed to dry.
THF is essentially the only reliable chemical base for removing parylene conformal coatings below the melting point. Some success has been achieved using either chloronaphthelene or benzolyl benzoate to affect parylene removal from substrates at temperatures above 150 degrees Centigrade. However, these chemicals are incompatible with most commonly applied parylene processes, offering only very limited use and minimal recommendation. Parylene's chemical inertness restricts chemical removal in virtually all cases.
Recommended Removal Techniques for Parylene Conformal Coatings
Mechanical: Care must be taken using any of the most prominent mechanical removal techniques. These involve cutting, picking, sanding or scraping the precise surface-expanse of coating to be removed. Because parylene's conformal coatings are exceptionally uniform, durable and resist manipulation, careless or inept application of any of these methods can result in damage to the assembly. Nevertheless, mechanical methods can generate very good results for spot-removal, areas where parylene needs to be removed due to:
- poor initial coating application, or
- the coating's interference with component functions, among other reasons.
Masking the surrounding area is always recommended.
Although little clean-up is required, cutting and related mechanical techniques are not reliable options for removing parylene from the entire surface area of any related parylene-coated assembly.
Plasma: Parylene can be readily removed through application of oxygen-based plasmas. For Parylene N, plasma removal is generated by opening of the substance's benzene ring; for Parylene C a similar reaction occurs, differentiated only by the absence of a chlorine atom. In either case the ring opens with introduction of an oxygen radical, causing:
- a hydroxyl radical to form between the benzene rings in the polymer chain,
- followed by molecular/atomic oxygen-absorption,
- generating an unstable peroxy radical, subsequently
- rearranged into either volatile carbon monoxide or carbon dioxide.
Further plasma manipulation on the radical site expands the ring-opening, accelerating parylene removal.
Other Dependable Methods of Removing Parylene
Abrasion: In many cases, this is the easiest and fastest method for removing parylene conformal coatings uniformly applied to substrate surfaces. Masking the area to be stripped is recommended for spot removal; the objective is to ensure a dependably clean surface-edge when parylene is reapplied.
Thermal: Thermal methods generate high quality deletion of parylene coatings, but their use should be restricted to spot-removal. Application to a whole PCB or similar appliance is not suggested, since their use beyond spot-removal can lead to much diminished control of the process and ruined coatings outside the target area.
Although highly popular for removal of most conformal coatings, chemical removal of parylene is not recommended. Parylene is resistant to dissolution by solvents. Only THF provides a reliable chemical basis for removing parylene coatings from assembly substrates; other options are highly specialized and seldom applied. Of more value are dependable mechanical and plasma-based techniques; these are particularly useful for spot-removal assignments. Abrasion techniques represent a popular removal option; thermal methods have also been successful spot-removing parylene.