Safely Removing Parylene Coatings

While conformal coatings are known for reliably safeguarding the surfaces of printed circuit boards (PCBs) and associated electrical components, issues may arise that necessitate their removal. Chemical removal is suitable for wet coating substances such as acrylic, epoxy, silicon and urethane, as it causes minimal damage to PCBs. However, when it comes to Parylene, chemical removal methods are less effective.

Parylene’s chemical vapor deposition (CVD) process plays a pivotal role in the coating’s distinctive property advantages that set it apart from wet coatings. Unlike liquid methods such as immersion and spray used by acrylic, epoxy, silicone and urethane, Parylene offers substantial application benefits. Wet coating techniques are constrained by surface tension and gravitational effects, hindering uniform coverage across all surfaces. In contrast, CVD facilitates consistent, pinhole-free, hermetic and homogeneous coating of all surfaces with gaseous Parylene, even reaching into the tiniest corners, edges and ripples. These attributes establish Parylene as an optimal conformal coating for critical applications in aerospace, medical, automotive and microelectromechanical systems (MEMS). In addition to excellent film uniformity, Parylene also provides:

• Biocompatibility
• Chemical resistance
• Electrical isolation
• High dielectric strength
• Hydrophobicity
• Low friction
• Minimal permeability to gases
• Optical transparency
• Thermal stability

However, Parylene applied using CVD is much less susceptible to chemical removal compared to its wet counterparts.

Tetrahydrofuran (THF), a Chemical Removal Solution for Parylene?

Parylene is chemically inert. This property effectively negates the usefulness of the liquid chemical removal methods that work for the majority of conformal coatings. The one chemical that has been successfully used to strip Parylene from substrates is tetrahydrofuran (THF), a colorless organic compound whose chemical formula is (CH2)40. It demonstrates low viscosity at standard pressure/temperature and is water-miscibleorganic.

Duration of use of THF for removal is largely dependent on the thickness of the Parylene film. For example, a Parylene thickness of .001 mm requires immersion between 2-4 hours in a THF-based solvent. The Parylene coating begins to separate from the assembly’s surface during immersion. After rinsing the assembly in alcohol and ensuring it is thoroughly dry, the Parylene film is removed from the surface of the assembly using tweezers.

Other than THF, the only other chemicals that have successfully removed Parylene coatings are benzolyl benzoate and chloronapthelene, at temperatures above 150°C. However, these chemicals are minimally effective in removing Parylene films due to their general incompatibility with most Parylene processes. It is recommended not to use them unless for highly specific scenarios.

The exceptional chemical inertness of Parylene limits its removal through chemical means in nearly all scenarios. Therefore, alternative removal methods must be utilized to guarantee thorough elimination of the coating and to safeguard the integrity of the components beneath the Parylene film.

Reliable Methods of Removing Parylene Conformal Coating

Abrasion

Expeditious and cost-effective, micro-abrasive removal of Parylene films are easy to implement and environmentally friendly. Micro-abrasive blasting propels explicit formulas of inert gas/dry air and abrasive media at the Parylene-coated component, via a tiny nozzle attached to a stylus; either a handheld human or automated systems can be used to pinpoint the targeted removal area. Conducted within an enclosed anti-static chamber, a vacuum system persistently removes the Parylene debris from the substrate, with disposal implemented by filtration processes. Grounding devices dissipate electrostatic potential. Abrasion removes Parylene coatings from a single test node, an axial-leaded component, a through-hole integrated circuit (IC), a surface mount component (SMC) or an entire PCB. Abrasion is often the easiest and fastest method for removing Parylene conformal coatings uniformly applied to substrate surfaces.

Laser

Typically utilizing pulsed laser sources, laser ablation converts Parylene to gas or plasma. Control must be exercised, since each laser pulse separates only a tiny proportion of the film’s material thickness. Nevertheless, ablation is cost-effective for complex removal jobs, since processing can be enacted in a single step. Better quality removal results, with 100% Parylene-free areas; photo-ablation particularly delivers excellent outcomes for these purposes. Design compromise is lower than with other removal processes, since laser application can be controlled to a single micron. 3D devices can also be effectively serviced.

Mechanical

Effective removal of coatings through mechanical techniques such as cutting, picking, sanding, or scraping demands meticulous care and attention to detail. The exceptional uniformity of Parylene coatings combines with their capacity to withstand manipulation and overall strength to accelerate damage if mechanical processes are imprecisely applied. While appropriate masking can lead to good Parylene spot-removal, mechanical techniques are undependable for larger-scale surfaces.

Plasma

Application of oxygen-based plasmas can remove Parylene films. For Parylene C and N, plasma removal begins by opening the benzene ring through introduction of an oxygen radical, causing generation of a hydroxyl radical between the polymer chain’s benzene rings. Oxygen absorption at the atomic/molecular level follows, causing development of an unstable peroxy radical, subsequently rearranged into either volatile carbon monoxide or carbon dioxide. Parylene removal proceeds more quickly with additional plasma manipulation on the radical site, growing the opening in the substance’s benzene ring.

Thermal

The thermal Parylene coating removal technique (including using a soldering iron to burn through the conformal coating) is the least recommended technique of coating removal. Thermal is difficult to manage. Its use should be restricted to spot-removal; larger-scale removal can rapidly generate ruined coatings outside the target area, emanating from much diminished process control and emission of toxic vapors.

Summary

THF is the only chemical solvent that consistently provides reliable Parylene removal from assembly substrates; the limited chemical options remaining are highly specialized and seldom applied. Abrasion techniques represent a popular removal option; laser methods are expected to develop further as a major removal process for Parylene films. Mechanical and plasma-based techniques are useful for spot-removal assignments. Thermal methods also have some use for spot-removing Parylene but are difficult to control.