Parylene Coating Blog by Diamond-MT

Five Common Causes of Parylene Failure

Posted by Sean Horn on Fri, Aug 05, 2016 @ 08:00 AM

Parylene Conformal Coatings

Parylene application typically generates an ultra-thin layer of electrical insulation that prevents contact between physical contaminants and the conductive elements of printed circuit boards (PCBs).  Parylene also protects PCBs from the impact of corrosive vapors and moisture that may degrade their metallic components, providing a reliable barrier that significantly retards the impact of these contaminants.  

Parylene is differentiated from competing conformal coatings – acrylic, epoxy, silicone and urethane – by its application process.  Whereas the other conformal coatings employ some kind of wet application method – brushing, dipping or spraying are common methods -- parylene’s chemical vapor deposition (CVD) process has no intermediate liquid phase.  Beginning in a powdered state, the parylene is converted to a gas through cross-link polymerization at the molecular level; there is no intermediate liquid phase.  The resultant vapor penetrates deep with the substrate surface to provide absolutely conformal coating that typically surpasses the performance of wet coatings on most functional levels. 

While its distinctive deposition technique generates the majority of parylene’s functional advantages compared to competing coating materials, it can also be the source of parylene failure.  In all cases, proper application is essential to assure parylene’s optimal performance.  Otherwise, problems may emerge due to failure of the parylene film.  

Five Sources of Parylene Failure

Inappropriate parylene application can result when contaminants such as stray particles, oil, metal shavings, dust, and dirt remain on assemblies during the deposition process.  These disrupt parylene penetration of substrate surfaces and the creation of the uniform, pinhole-free coatings that are its best-known quality.   The consequent weakened parylene film-to-surface bond, can lead to such causes of parylene failure as (1) bubbles, moisture diffusion and pinholes, (2) corrosion, (3) cracking, (4) delamination, and (5) problems from substrate outgassing. Conformal-Coating-on-Printed-Circuit-Board.jpg

  • Moisture Diffusion and Surface Pinholes: Although parylene is highly regarded for its completely conformal covering, failure can occur if substrate surfaces are inadequately prepared.  The presence of organic or inorganic contaminants can lead to imprecise coatings, as contaminants become trapped beneath the applied film, interfering with the surface penetration that is characteristic of parylene.  In such cases, parylene’s ultra-thin coating can suffer from pinholes, diminishing its protective capacity.  In addition, parylene-metal-parylene thin films used for micro-electro-mechanical systems (MEMS) technology can exhibit corrosion behavior when subjected to exposure in saline after as few as two days.  Much depends on the thickness of parylene packaging.  When coatings are thin – less than 5 μm -- moisture can diffuse through the parylene under accelerated soak-conditions or exposure to high electric field environments.  Contaminants trapped inside parylene coatings lead to structural breakdown.  Parylene moisture barrier performance is improved by increasing the thickness of the parylene film.    

Conclusion

Because of vulnerability to contaminants, cleaning the surface of the substrate prior to coating is crucial to achieving parylene adhesion.  Trace contaminants disrupt the bond between parylene film and underlying surfaces.  Not always immediately apparent, compromised adhesion will eventually compromise the coating and, ultimately, the end product, thus neutralizing parylene’s protective benefits.

To learn more about how to improve parylene adhesion to limit failures, download our whitepaper:

Guide to Parylene Adhesion

Tags: parylene, parylene disadvantages, parylene issues, medical device coating, parylene adhesion, parylene delamination