Forty Years of Parylene Technology


Background

Para Tech Coating, Inc. (PTC) of Aliso Viejo, California, is preparing to celebrate 40 years of leadership in the commercial application of Parylene coating. The company was founded in 1968 by Jeffrey Stewart, an electrical and mechanical engineer with many years experience in magnetic, electronic component design and systems engineering. He became one of the first commercial licensees of Union Carbide Corporation's poly-para-xylylene film coating process, and developed this California firm into a major contributor to Parylene technology and in its protective coating applications and equipment since it was founded four decades ago.

Over the years, PTC's work with Parylene has included coating ferrite cores, military components, items destined for outer space, aerospace assemblies, disposable and implantable medical devices, sensors and keypads. Additionally, the firm's contributions to equipment design have provided significant benefits to coating uniformity and reduced process time and material consumption. PTC outgrew several locations in Orange County, California, eventually moving to dedicated facilities in Aliso Viejo in 1989 that still serve as its corporate headquarters.

While several original equipment manufacturers had experimented with Parylene for internal requirements in the 1960s, PTC was the first Union Carbide licensee to open a Parylene center as a commercial coating service. The firm's initial impetus for developing Parylene vacuum deposition capabilities was to coat ferrite cores and pressed powdered metal parts for pulse transformers and delay lines destined for computers and related applications.

"As a very thin and conformal film, Parylene offered important advantages over traditional varnish for coating ferrites," Stewart explains. "Treatment with varnish changed magnetic core properties and necessitated that cores be graded. Parylene eliminated this step in pulse transformer manufacture.

"Additionally, it improved coil winding properties, added dielectric protection and reduced wire abrasion while adding very little physical mass," he said. It was soon apparent that these and other Parylene characteristics made the coating uniquely suitable for sensors and many other conformal coating applications, including electronic components, printed circuit boards and various substrates. Thus a number of Parylene uses were developed and refined during the earliest days of the company.

Parylene History

The Parylene molecule was isolated by a researcher at the University of Manchester, England, in 1947. Film developed at this laboratory was the world's first vapor deposited poly-para-xylylene or PPX, which in purer colorless form is known today as Parylene N.

This work inspired additional research in several industrial laboratories, and a few years later a Union Carbide scientist developed a simpler means of producing the dimer that is the raw material from which Parylene film is deposited. Union Carbide tested many chemical alternatives and introduced a commercially viable Parylene coating system in 1965 that consisted of a family of three polymers (Parylenes N, C and D) and a vacuum deposition method.

Coating Methods

Protective industrial coatings have a number of purposes, including providing mechanical or electrical isolation, serving as a barrier to chemicals, moisture and gas, and immobilizing tiny particles. Various industrial coating materials were in common use before the development of Parylene, and there have been continued developments in conventional coatings such as epoxies, silicones, acrylics and urethanes over the intervening years.

Each of these coating materials exhibits properties that suit it to particular applications having unique thermal, chemical, electrical and mechanical requirements. However, unlike Parylene, the listed materials are liquid in nature and thus subject to phenomena such as meniscus, pooling and flow. Each of them creates a degree of potentially damaging mechanical stress on coated surfaces during cure and subsequent thermal cycling. The typical thickness of a liquid coating ranges from 0.005 to 0.010 inches.

Since thin Parylene film is formed by heating a powdered raw material to form a gas that ultimately polymerizes on surfaces in a room-temperature vacuum chamber, it has no liquid phase. The film is absolutely conformal and pinhole free, and builds with equal thickness on all surfaces, including over edges and in crevices. There is no cure phase and no mechanical stress associated with Parylene. It can be applied in layers as thin as 0.1 micron or as thick as 75 microns in a single pass, with a typical rate of film buildup of approximately 5 microns/hr.

PTC Contributions to Parylene Technology

The practical application of Parylene to ferrite cores, circuit assemblies and other substrates by PTC led to observation of certain limitations and deficiencies in the conventional coating method.

"One issue was coating system inflexibility," Stewart notes. "Small devices such as ferrite cores need to be continuously rotated or tumbled in the coating chamber for the polymer to form evenly on all surfaces. But circuit boards and other larger assemblies must be masked and fixed in place for coating."

In response, Jeffrey Stewart and his team developed a custom Parylene coating machine that could be used horizontally for rotating small, loose parts, and quickly converted to the vertical position for coating larger fixed components.

The PTC founder recalls that early deposition systems had no means of control for the heating unit that converts the powdered dimer raw material to gaseous form. As a result, the ultimate film properties achieved in individual coating cycles were only a rough approximation of the specified target.

"In the early and some current systems a pressure feedback concept was and is used to control dimer temperature as it is heated to achieve a gaseous state, but this negative feedback method creates a hysteresis effect that makes it difficult to accurately maintain the set target level," Stewart explains. "With this approach, the control system is always chasing the target temperature rather than achieving and maintaining it. The fact that the dimer is heated in a vacuum adds to the temperature control lag."

PTC altered this control system to establish a set rate of incremental increase in loading zone heat, to monitor the resulting pressure, and to determine when to adjust heat input incrementally by means of a programmed controller. As a result PTC's method of controlling the deposition process, became much more predictable, and target film values could be achieved consistently.

Stewart also recalls that early coating chamber designs were inefficient in that the mechanical configuration prevented linear movement of the vaporizing monomer across the chamber load, resulting in uneven coating levels. He devised and patented a tangential entry designs that allows the polymerizing gas to flow evenly around and across the chamber. The chamber exit at the opposing side of the chamber is also tangential, augmenting even flow and improving the efficiency of the cold trap, which likewise uses a tangential design that captures and condenses unused monomer more efficiently.

PTC did early and effective work in the area of fixture rotation, determining the optimum speed of the fixture turntable in the coating chamber to minimize vortex and monomer turbulence. Additionally, a proprietary PTC tumble coating approach causes parts to traverse an angular eccentric path, moving slowly along the chamber in both directions and minimizing abrasion while improving coating uniformity.

Over the years Jeffrey Stewart developed and patented eight useful improvements in the Parylene deposition process that dramatically increased the performance, consistency and reliability of the Parylene vacuum deposition process. These PTC innovations continue to distinguish PTC as an industry leader, setting the standard for Parylene deposition efficiency and precision in the company's coating centers as well as in PTC coating systems designed and built for on-site customer use.

Building Trust

Many Parylene coating applications involve costly, sophisticated and often proprietary substrates that must be diverted from an OEM's production stream for handling at PTC. This process demands that the Parylene provider function as a trusted outside partner, maintaining just-in-time scheduling, absolute quality assurance and industrial security in full accordance with individual customer requirements. PTC established and has maintained this level of trust over four decades of service through a commitment to every required performance standard.

Intensive review by an accredited external auditor found PTC's Quality Management System to be compliant with demanding best practice standards, and on October 18, 2006, PTC's headquarters operation was officially certified to the internationally-recognized quality standard of ISO 9001:2000. The ISO certification relates to all aspects of the business, including production, installation and service, as well as ongoing certification compliance. This ISO certification confirms that all internal PTC processes and protocol will meet and exceed customer quality expectations.

Since Parylene has been widely accepted for the most challenging coating applications, military and IPC standards include specifications that pertain specifically to Parylene as well. PTC continuously meets these requirements.

"It has always been our practice to invite prospective customers to the coating center floor to review our processes, help with establishing any necessary practices, and participate in developmental runs," Stewart says. "We earned our reputation early-on for taking good care of customer parts, maintaining rigorous procedures and assuming all of the responsibilities of a long-term business partnership. Those practices have continued over the years, and PTC continues to enjoy a strong reputation as a quality Parylene coating provider and dependable business collaborator."

Expanding PTC Service

In 1999, PTC opened a coating operation in Middletown, Connecticut, to more directly serve coaters of ferrite cores on the East Coast. This office originally began as a regional sales effort, but soon expanded to a full service custom coating center with coating equipment sales and support. Today PTC's East Coast operation offers all of the same services and meets the same demanding performance standards as the company's headquarters facility. PTC customers in the Midwest are served by a Minneapolis, Minnesota sales office.

In the 1990s, the company began to develop international operations, with its first joint venture opening in Korea in 1994. In 1997 PTC formed a joint venture with a partner in Sweden to serve the European market, and more recently developed a business partnership in China to represent the company in the Far East.

Each of PTC's international operations receives training, system and procedure support and Parylene application engineering from the company's Aliso Viejo, California headquarters. All locations operate with the same emphasis on quality control, customer support and a commitment to building absolute trust among co-workers, customers and suppliers in order to meet the highest performance standards.

Conclusion

"Parylene is unique in the realm of conformal coating," Stewart concludes. "While the basic technology is essentially unchanged after forty years, Parylene is used today in critical coating applications that were unimagined in the 1960s before the advent of medical implants, micro-electronics, nanotechnology and sophisticated aerospace and industrial applications."

Process knowledge and application engineering expertise continue to be valuable assets as a growing number of manufacturers look to Parylene and Para Tech Coating, Inc. to meet challenging coating requirements. With four decades of leadership in process refinement and a success with a myriad of coating applications, the company is uniquely equipped to serve as an effective partner in delivering conformal coating solutions for years to come.

Reproduction quality photos are available upon request from: tomdewey@deweycomm.com

5411 Mt Normandale Dr.
Bloomington, MN 55437
952-831-2192

Contact:
Anita Brown, Marketing Manager
Para Tech Coating, Inc.
35 Argonaut, Aliso Viejo, CA 92656
(800) 999-4942 ext. 875 - abrown@parylene.com

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