Solvent Resistance of Parylene

Parylene variants offer robust solvent resistance, protecting substrates like printed circuit boards (PCBs) and other electronics components. This high level of protection is sustained over a wide temperature range that exceeds typical operating conditions for PCBs. The exceptional solvent resistance of Parylene is attributed to its unique molecular structure, which makes it largely insoluble in organic solvents and resistant to strong inorganic acids and bases.

Unlike liquid coatings – acrylic, epoxy, silicone, urethane – which employ brush/dip/spray application methods, Parylene’s exclusive vapor deposition polymerization (VDP) process adds value to Parylene conformal films. Coating thicknesses ranging from sub-micron to several thousandths of an inch can be applied in a single deposition run; unique to VDP, this property fosters the pinhole-free uniformity of Parylene coatings. Another result of vapor-phase polymerization is the distinctive molecular structure of Parylene protective films, which penetrate deep into substrate surfaces. Chemically, the various Parylenes’ main-chain phenyl group generates reliable molecule-to-molecule interaction, while possessing high levels of in-plane electronic polarization capabilities. The chemically stronger film consistency, in comparison to liquid-derived and assembled monolayers, is the source of Parylene’s chemical/solvent resistance.

Solvent Impact Higher Than 150°C

Several solvents impact Parylene films above 150ºC:

• Chloro-naphthalene (C10H7Cl) softens and slightly dissolves Parylene C coatings at temperatures of 175ºC and higher.
• The solvent benzolyl benzoate (BnBzO) has a similar effect on Parylene N films at the considerably higher temperature of 265ºC.

Nevertheless, these chemicals remain incompatible with most current practical Parylene processes and are infrequently applied for conformal coating removal. Parylene exhibits resistance to permeation by most solvents. In addition, such stress-cracking agents as “Hostepal,” “Igepal” and lemon oil have no effect on the solubility of Parylene coatings.

Organic solvents – including acetone, chlorobenzene, isopropyl, o-dichlorobenzine, pyrindene, trichloroethylene and trichloro-trifluoroethane – have minimal effect on Parylene conformal films. Their presence causes minor swelling effects on Parylene, with increased film thickness maxing-out at 3%. These effects are entirely reversible; once the solvents have been removed, Parylene films revert to their previous condition.

For inorganic solvents like non-oxidizing hydrochloric or sulfuric acid, or oxidizing acids like chromic or nitric acid, slight swelling – less than 1% – is detectable at room temperature, in most cases. At 75°C for 2 hours, Parylenes N and C can swell beyond 5% in oxidizing chromic acid, as can type D in non-oxidizing sulfuric acid. Sustaining these levels of exposure can lead to severe degradation over time.

Solvent Tetrahydrofuran (THF)

Only water-miscible THF ([CH2]40, oxolane), a polar aprotic solvent, softens Parylene coatings for potential removal from substrates. For instance, at a thickness of .001 mm, THF solvent can detach Parylene from a substrate, following 2 – 4 hours of immersion. After immersion initiates Parylene separation, the assembly is removed from the THF solution, rinsed in alcohol and allowed to dry. The Parylene coating is then lifted from the assembly’s surface with tweezers or a similar tool.
This procedure is not a preferred solution since THF is a known carcinogen and its use must be carried out with extreme caution.

Conclusion

Chemically inert, the Parylenes are exceptionally resistance to solvents at room temperature, a property that is maintained up to elevated temperatures. Therefore, they remain relatively unaffected by solvents’ presence or potential impact. In contrast, solvents can delaminate, erode or otherwise destroy the effectiveness of liquid coatings. Parylenes also provide effective barriers to other corrosive agents.
Parylene solvent-resistance contributes to the formation of truly conformal substrate coatings that sustain assembly protection and performance for the duration of its expected operational life. Visit the SCS Technical Library to download our white paper on solvent resistanc