Protecting aerospace components: QPL verification is a key in Parylene coating specification

Vacuum-deposited film has been used for decades to protect surfaces and components in aerospace electronics. The Parylene formulation is molecularly specific, whereas the Parylene dimer--the raw form of the coating--varies in purity and performance from one manufacturer to another. (See "A Parylene conformal coating overview" for more.) A deficient dimer formulation will degrade the ability of finished Parylene film to protect critical assemblies under harsh conditions.

Thin, transparent Parylene film has been lifted from a test coupon for digital micrometer measurement and other rigorous tests as part of the QPL certification process.

Manufacturers of critical systems are threatened by the possibility that a subcontractor might substitute counterfeit or fraudulent components or materials for Mil-Spec assemblies and other high-risk devices. Such unauthorized and potentially undiscovered substitutions threaten the performance and ultimate reliability of such assemblies.

A chemically based material such as Parylene dimer presents a particular risk, since it is very difficult to detect an inferior, non-QPL (Qualified Product List) material apart from destructive testing, and effective coating encapsulation is a key component in system reliability. The Defense Logistics Agency manages QPL operations through its Defense Supply Center in Columbus, Ohio (www.landandmaritime.dla.mil).

Parylene film is superior to liquid coatings in encapsulating and protecting delicate electronic circuits, yet it adds almost no mass, and is conforms absolutely to flat surfaces, around edges, and into crevices.

The QPL certification program is intended to assist design engineers and circuit manufacturers with the complex task of analyzing and qualifying products and materialsm such as Parylene dimer, to ensure they conform to standards and meet the high-performance requirements of aerospace, defense, medical, and other critical electronics applications. Today, there are more than 30,000 products or components in eight categories that are covered by the ongoing QPL process.

Products submitted for QPL review are subject to extensive testing against detailed performance and quality requirements, and then validated by subsequent audits on a continuing basis. Products that achieve QPL status are listed by Defense Logistics Agency Land and Marine in a qualified product database that is subject to ongoing additions, deletions and modifications.

While non-QPL-rated Parylene dimers may be sufficient for routine, non-critical coating tasks, there is no assurance that they meet the performance level required for the most demanding coating applications since they have not been subject to QPL testing.

There may be no apparent difference in appearance between dimers of various suppliers, nor in their functionality for routine vacuum coating. Dimer performance can, in fact, only be determined by means of the thorough, sophisticated laboratory analysis involved in QPL testing. Higher-performance, QPL-qualified Parylene raw material may be more expensive than non-conforming dimers, but substitution of an inferior, non-QPL material compromises the performance of sophisticated, mission-critical components, with potentially severe and costly consequences.

It is crucial, therefore, for system specifiers and subcontracting decision makers to insist on the documented use of a QPL-listed Parylene dimer, along with optimal substrate cleaning, surface preparation, and coating deposition procedure, in order to meet governing regulations.

The QPL program is a valuable resource for manufacturers as they identify and select a Parylene coating resource, as well as to those companies with in-house Parylene coating operations.

Para Tech Coating Inc. (Para Tech) supplies QPL-rated Parylene raw materials that are approved per QPL-46058, as specified for products under MIL-I-46058. Para Tech is one of few U.S. providers of QPL-rated Parylene dimer to coating service customers and in-house Parylene coating operations, as well as to other Parylene coating suppliers.

The QPL process
The QPL listing process begins with the coating of test coupons using the dimer under review and submitting them to an authorized third-party laboratory for detailed analysis of physical and chemical properties, in accordance with MIL-I-46058C Standard guidelines, Para Tech Quality Control Manager Patricia Langraphi explains. The results of these tests are submitted to the Defense Logistics Agency (DLA) for review and approval; with DLA authorization, the material is listed on the QML/QPL database.

Associated quality issues
The QPL rating program for dimer supports the broader electronic manufacturing quality requirements of customers following the AS9100 standard. This standard incorporates the full scope of the original ISO 9000 program, along with additional requirements specifically related to aerospace and defense industries. As QPL approval is an essential requirement for raw material used on defense projects, QMS certification based on aerospace standard AS9100 is an essential requirement for companies doing business with aerospace and defense organizations.

In 2008, Para Tech’s headquarters in Aliso Viejo, Calif., became the first Parylene coating service facility in the country to achieve AS9000 QMS certification, and was subsequently upgraded to AS9100 Rev. C in 2011. In May 2012, Para Tech’s Middletown, Conn., facility also achieved QMS certification based on AS9100 Rev. C.

“Our quality management system is based on a robust system, where AS9100 Rev. C requirements and quality principles are the foundation for all of our processes. Every aspect of our operation has been adjusted and refined to conform to the latest standard,” Langraphi notes. “We maintain certification based on a three-year cycle that involves verification of every individual process and of the entire system. A certification authority conducts periodic on-site audits to verify conformance of the QMS. The inability to demonstrate compliance to the standard requirements can result in major or minor non-conformance findings, and the possibility of suspension of certificate.

“The results of periodic AS9100 audits are published in the Oasis database, which is open to review by coating service customers and prospects. Both our East Coast and West Coast Parylene coating facilities must operate on the basis on constant attentiveness to every aspect of quality in order to maintain the highest level of quality assurance. This is the standard of daily performance at Para Tech,” Langraphi concludes.”

Conclusion
The QPL rating of Para Tech dimer completes the circle of AS9100 requirements for competent vacuum deposition coating of electronic systems for the highest level of performance and customer peace of mind. This is a process that few providers are equipped to provide.

Well-meaning, established, and otherwise competent companies may inadvertently use counterfeit materials or processes, and it is imperative that every link in the supply chain pays close attention to detail to ensure compliance with materials and methods requirements.

If the quoted cost for a system element seems too good to be true, investigate the supplier closely to make sure that the quotation covers and verifies QPL listings, along with every other aspect of production quality.