Conformal Coating – Conformal coating material is a thin polymeric film which ‘conforms’ to the contours of a printed circuit board to protect the board’s components. Typically applied at 25-250 μm it is applied to electronic circuitry to act as protection against moisture, dust, chemicals, and temperature extremes that, if uncoated (non-protected), could result in damage or failure of the electronics to function.
Reasons for use of Conformal Coating
Conformal coatings are used to protect electronic components from the environmental factors they are exposed to. Examples of these factors include moisture, dust, salt, chemicals, temperature changes and mechanical abrasion. Successful conformal coating will prevent the board from corroding. More recently, conformal coatings are being used to reduce the formation of whiskers, and can also prevent current bleed between closely positioned components. Conformal coatings are breathable, allowing trapped moisture in electronic boards to escape while maintaining protection from contamination. These coatings are not sealants, and prolonged exposure to vapours will cause transmission and degradation to occur. There are typically four classes of conformal coatings: Acrylic, Urethane, Silicone, and Varnish. While each has its own specific physical and chemical properties each are able to perform the following functions:
- Insulation: Allowing closer conductor spacing.
- Eliminate the need for complex enclosures.
- Minimal effect on component weight.
- Completely protect the assembly against chemical and corrosive attack.
- Eliminate performance degradation due to environmental hazards.
- Minimize environmental stress on a PCB assembly.
Parylene is the generic name for members of a unique polymer series. These members (or variations of Parylene) each offer their own, slightly different, coating properties to engineers. Commercially available Parylene variants, along with their respective properties, include:
The basic member of the series, Parylene N, is poly(para-xylylene), a completely linear, highly crystalline material. Parylene N is a primary dielectric, exhibiting a very low dissipation factor, high dielectric strength, and a low dielectric constant invariant with frequency. The crevice-penetrating ability of Parylene N is second only to that of Parylene HT.
Parylene C, the second commercially available member of the Parylene series, is produced from the same raw material (dimer) as Parylene N, modified only by the substitution of a chlorine atom for one of the aromatic hydrogens. Parylene C has a useful combination of electrical and physical properties, plus a very low permeability to moisture and corrosive gases.
Parylene D, the third member of the series, is produced from the same raw material as Parylene N, modified by the substitution of chlorine atoms for two of the aromatic hydrogens. Parylene D is similar in properties to Parylene C with the added ability to withstand slightly higher use temperatures.
Parylene HT, the newest commercially available variant of Parylene, replaces the alpha hydrogen atom of the N dimer with fluorine. This variant of Parylene is useful in high temperature applications (short term up to 450°C) and those in which long-term UV stability is required. Parylene HT also has the lowest coefficient of friction and dielectric constant, and the highest penetrating ability of the four variants.
Parelene Deposition Process
Parylene coatings are applied at ambient temperatures with specialized vacuum deposition equipment. Parylene polymer deposition takes place at the molecular level, where films essentially ‘grow’ a molecule at a time:
A solid, granular raw material, called dimer, is heated under under vacuum and vaporized into a dimeric gas.
- The gas is then pyrolized to cleave the dimer to its monomeric form.
- In the room temperature deposition chamber, the monomer gas deposits on all surfaces as a thin, transparent polymer film.
Because Parylene is applied as a gas, the coating effortlessly penetrates crevices and tight areas on multi-layer components, providing complete and uniform encapsulation. Optimal thickness of the polymer coatings is determined based on the application and the coating properties desired. While Parylene coatings can range in thickness from hundreds of angstroms to several mils, a typical thickness is in the microns range.
Factors affecting Parylene Coating Process
- Substrate Material
- Substrate Cleanliness
- Substrate Penetration
- Repeatable Coating Process
Parylene Application Evolution
When Parylene was first commercialized, it was used as a conformal coating for the small fritter rings used in core memory, but as technology advanced, Parylene expanded into protecting premium printed circuit boards in the electronics industry. Over the years, as industry expanded with innovation after innovation so, too, did the use of Parylene.
A perfect fit for Aerospace
Parylene’s high altitude/vacuum performance started gaining attention within the aerospace industry in the 1970’s. Parylene’s inherent barrier properties were important, but even more so was the nature of its vacuum deposition process (VDP). Being deposited in a vacuum meant the Parylene trapped no air in or on a coated device or component. In high altitude/lower pressure environments, this trapped air can expand and potentially rupture the coating. The result is not only a destroyed device but also the potential for a destroyed mission as well. Parylene also has higher temperature capabilities in the absence of oxygen — the vacuum of deep space.
Early in its commercialization, Parylene was being reviewed by a number of forward thinkers including those in the medical device industry. They started to recognize Parylene’s ability to provide unique performance capabilities in very thin coatings. Even more intriguing was the discovery that Parylenes’ N and C were inherently biocompatible. Today, Parylene is used in and on pacemakers as it has for nearly 40 years. Parylene is also now being used on coronary and cerebral stents. This includes drug-eluting stents (DES) that feature drug/polymer combinations. In DES applications, the Parylene serves as a critical tielayer or primer that enables the drug/polymer combinations to be integrated where normally they would not.
Application in MEMS
One of the most recent applications to take advantage of Parylene is the world of Micro electro mechanical Systems (MEMS) and Motes. The micro- and Nano- dimensions of these devices demand a coating that provides ultra-thin, microscopic uniformity — a feature not available with coatings that are applied via brush, spray or by dip process.
Parylene in 3-D Applications
While Parylene conformal coatings continue to expand in many industries and applications, we are also entering a new phase in the evolution of Parylene applications. Generically referred to as 3-dimensional applications, this new frontier is now being perused by researchers who are working to create 3-D structures made entirely of Parylene. These devices fall into the category of MEMS, and promise a new wave of commercialization of Parylene.