Despite conformal coatings’ ability to dependably protect substrate surfaces of printed circuit boards (PCBs) and related electrical components, problems can sometimes occur which compel their removal. Chemical removal, which does the least damage to PCBs, is fine for wet coating substances like acrylic, epoxy, silicon and urethane. Chemical removal methods are far less successful for parylene, despite the use of a chemical vapor deposition (CVD) process for its film application.
CVD processes generate many of the property advantages that distinguish parylene from wet coatings. Parylene offers significant application advantages in comparison to the liquid methods – immersion, spray, etc. – used by acrylic, epoxy, silicone and urethane. Surface tension and gravitational influences effect wet coating methodologies, limiting the capacity to evenly cover all component surfaces. CVD generates uniform, pinhole-free, hermetic and homogeneous coverage of all surfaces with the gaseous parylene, including the smallest corners or crevasses, pointed edges or surface ripples. These properties position parylene as an ideal conformal coating for critical uses in aerospace, medical and microelectricalmechanical systems (MEMS) applications.
In addition to excellent film uniformity, parylene also provides:
- chemical resistance,
- high dielectric strength,
- low friction,
- minimal permeability to gases,
- optical transparency, and
- thermal stability.
But CVD-applied parylene is far less amenable to chemical removal than its wet competitors. There is one major exception.
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:
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 subsequent thorough drying, the parylene film is then removed from the assembly’s surface, physically, with tweezers.
Other than THF, the only other chemicals that have successfully removed parylene coatings are benzolyl benzoate and chloronapthelene, at temperatures above 150 degrees Centigrade. However, these chemicals offer only very limited use for removal of parylene films, since they are essentially incompatible with the majority of parylene processes; except for highly specialized cases, their use is not recommended.
Parylene's chemical inertness restricts chemical removal in virtually all cases. Thus, other removal processes should be employed to ensure complete coating-removal and the security of components underlying 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. 3-D devices can also be effectively serviced.
Mechanical: Most mechanical removal techniques -- cutting, picking, sanding or scraping the precise surface-expanse of coating to be removed – require considerable care and attention. 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 application can rapidly generate ruined coatings outside the target area, emanating from much diminished process control and emission of toxic vapors.
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.
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