Parylene Solvent Resistant Conformal Coating

Solvent Resistance of Parylene

The parylenes resist attack and are insoluble in all organic solvents up to 150ºC. Parylene C can be dissolved in chloro-naphthalene at 175ºC and parylene N is soluble at the solvent boiling point of 265ºC. Both polymers are resistant to permeation by most solvents with the exception of aromatic hydrocarbons. They are also unaffected by stress-cracking agents such as “Hostepal,” “Igepal” and lemon oil.

The effect of a wide variety of organic solvents on parylenes N, C and D was investigated.

Six classes of organic solvents were examined: alcohol (isopropyl), ketones (acetone and 2,4-pentanedione), aliphatic hydrocarbon (iso-octane), aromatic hydrocarbon (xylene), chlorinated olefin (trichloroethylene), chlorinated aromatic (chlorobenzene and o-dichlorobenzine), heterocyclic base (pyridene), and fluorinated solvent (trichlorotrifluoroethane).

These solvents had a minor swelling effect on the parylenes with a 3% maximum increase in film thickness. The swelling was found to be completely reversible after the solvents were removed by vacuum drying.

Some inorganic reagents were also examined. These included: deionized water; 10% solutions of sodium hydroxide and ammonium hydroxide; non-oxidizing acids, hydrochloric and sulfuric, in concentrated and 10% solutions; and oxidizing acids, nitric and chromic, concentrated and 10% solutions.

The diluted inorganic reagents had little effect on the parylenes. The acids at 10% concentrations had virtually no effect at room temperature and, except for chromic, no effect at 75ºC. Concentrated acids at room temperature (23ºC) had little effect. Under severe conditions, 75ºC for 30 minutes, all acids had a measurable effect ranging from 0.7% swelling with hydrochloric to 8.2% with chromic. Additionally, nitric acid under these same severe conditions caused severe degradation. Both concentrated nitric and sulfuric acids caused some discoloration.

Earlier experience had indicated that parylenes N, C and D were insoluble in all common solvents. Parylene C, it was found, could be dissolved in high boiling liquids such as X-chloronaphthelene or benzoyl benzoate at temperatures above 150ºC. However, these solvents are seldom encountered in the electronic industry. Of greater importance are those solvents and reagents used in processing, especially in cleaning of components and assemblies. This study was undertaken to quantitatively measure the effect of the more common solvents and reagents on the parylenes. Included also were certain hydrocarbons serving as models for fuels. The organic liquids chosen for this study were selected as being representative of typical solvent classes. The protection provided to the coated substrates by the parylenes was also observed.

It was also known that the parylenes were inert to inorganic reagents (except for oxidizing agents). This conclusion was confirmed by the work reported here and quantitative results given.