Parylenes - Protection by Polymer

Topics Covered

Introduction

Chemistry and Deposition

Types of Parylene

Parylene N

Parylene C

Parylene D

Parylene HT

Properties of Parylenes

Parylenes in Medical Applications

Parylenes in the Electronics Industry

Introduction

An organic polymer coating that has been enjoying widespread popularity in America, notably for stabilising artefacts salvaged from the Titanic, is finding increasing use in Europe. ‘Parylene’, which is applied from the vapour state and protects against both environmental and electrical influences, is typically used on electronic assemblies, sensors, medical devices, aerospace and avionics equipment.

Parylene is the generic name for coatings produced from polymers of the para-xylylenes, compounds which have been available commercially for over 25 years but have only recently been finding increasing popularity in Europe. Parylene coatings are used when a combination of protection challenges needs to be addressed. They offer uniformity and completeness of coverage as well as good physical, electrical and chemical protection. Because no solvents are involved in the coating process it is unaffected by current legislation concerning volatile organic compounds (VOCs), ozone depleting chemicals or materials that may contribute to global warming.

The coating process is as follows. After thorough cleaning to remove soils, oils and ionic contaminants, items to be coated are treated with an organo-silane adhesion promoter. Areas not requiring coating are masked and the items placed in the coating chamber.

Chemistry and Deposition

The Parylene precursor, a white dimeric powder, is first vaporised at approximately 150°C and 135Nm^-2 vacuum. The vapour then moves slowly into a pyrolysis chamber at 680°C and 65Nm^-2, where it is converted into the monomeric diradical para-xylylene, which is thermally stable but kinetically unstable with respect to homopolymerisation. The highly active monomer vapour then enters the deposition chamber, which is at ambient temperature and under a vacuum of 15Nm^-2, where it condenses on all exposed surfaces and polymerises to a clear film of poly-(paraxylylene).

The thickness of the polymer film increases at approximately 0.2µm per minute for Parylene ‘C’ and at a slower rate for the ‘N’ grade. Film thickness is controlled solely by exposure time, and can be checked if required by using `witness strips' or glass slides.

Because they are deposited by vapour condensation and under tight vacuum, Parylene films are free from the defects and problems associated with liquid coatings, such as pinholes and the tendency to bridge and fillet when drying and curing. Since the deposition occurs at room temperature, stresses generally experienced with liquid coatings do not occur with Parylene, and problems associated with differential coefficients of expansion/contraction of the construction materials of coated items are similarly absent.

Types of Parylene

Parylene N

There are three common forms of Parylene. Parylene N is the polymer of para-xylylene. It has the greatest penetrating power with respect to deep recesses and ‘blind’ holes. It has a low dielectric constant which is independent of frequency, and a low dissipation factor, making it ideal for high frequency applications.

Parylene C

By incorporating one chlorine atom into the xylylene ring a second variant, Parylene C, is produced. This Parylene exhibits very low permeability to moisture and corrosive gases, and has a substantially faster rate of deposition than the N grade, but crevice penetration is by no means as good.

Parylene D

Parylene D, the third version, has two chlorine atoms in the ring and possesses superior physical and electrical properties at higher temperatures than the N and C grades. It also has the highest degree of thermal stability of the three variants.

Parylene HT

There is a final variant, Parylene HT, which has fluorine atoms in the ring and consequently has a higher resistance to elevated temperatures. Its particular properties make it of great interest in the avionics and semiconductor fields.

Properties of Parylenes

The many useful properties of Parylene coatings, tables 1 and 2, make them suitable for many different applications, and new uses are always emerging, of which medical applications are a good example.

Table 1. Parylene properties by polymer type.

Table 2. Primary Parylene coating functions for selected medical substrates

Parylenes in Medical Applications

When foreign objects and materials come into contact with human body tissue long-term resistance of these components to corrosive body fluids, electrolytes, proteins, enzymes and lipids is essential. Biomedical surfaces may require a protective coating to provide physical isolation from moisture, chemicals and other substances and to immobilise microscopic particles. They may also need passivation, electrical insulation and coating to reduce friction.

A crucial issue for producers of medical implants and surgical components is that of chemical inertness, since mechanical performance may be compromised by biostability issues. Medical materials that are not intrinsically biostable must be separated from the human body by an isolating material. Products such as bone pins, needles, medical probes, cochlear implants, catheters, cardiac pacemakers and general prosthetic hardware have to be biostable in order to prevent their degradation and reduction in medical efficacy.

Most solvent-based liquid coatings such as silicones, acrylics, epoxides, polyesters and urethanes may not meet toxicity and/or biocompatibility requirements and cannot be applied with precise control. On the other hand, because they are formed from a pure molecular precursor (a monomer vapour), transparent Parylene films have no contaminating inclusions, do not `out gas' and form effective barriers against the passage of contaminants from substrates to both the body and to the surrounding environment, and vice versa.

Areas in which the use of Parylene coatings may be of benefit include cannulae requiring dielectric insulation and precise coating thickness, medical seals that demand lubricity and inertness with minimum change to dimension and durometer values, and guidewires and stylettes that require lubricity, inertness and a coating that will not flake off.

Using Parylene can ensure catheters have a consistent coating thickness over their widely varying geometries, safeguarding lubricity and inertness. Stents requiring a biocompatible surface which has minimal impact on dimensions and physical and mechanical properties and blood pressure transducers needing precise coating thicknesses across their minute dimensions are further important uses.

Parylenes in the Electronic Industry

As well as the wide range of medical applications the trend towards packing more and more electronics functions into a smaller and smaller substrate area has lead to another wide market for Parylene. Intense demands are placed upon the inventiveness and ingenuity of both designers and manufacturers and the combination of new substrate materials, advances in interconnection technology, and packaging of semiconductor devices with other technologies such as flexible circuits has raised a number of peripheral problems. For instance, the mechanisms that degrade electronics and electrical assemblies working in ‘harsh’ environments include corrosion, electrochemical metal migration between conductors and crack formation in solder joints.

Four elements are necessary for metallic corrosion in the electronics field, water (as an extremely thin moisture film on metal surfaces), oxygen, the metal itself and some type of ‘driving force’ (an applied voltage or dissimilar metal contact). The more of these elements that can be eliminated from the surfaces of electronics assemblies, the lower the likelihood of degradation and corrosion. An extremely thin Parylene coating effectively locks out water vapour and oxygen from the coated surface, protecting circuits in those harsh environments.

Ionic contaminants are usually present in the microcratered surfaces of printed circuit boards. These may be residues of soldering fluxes, so-called 'fusing fluids', and fingerprints, all left behind because of ineffective post assembly cleaning. Under the combined influence of an applied voltage and condensed moisture vapour, electrochemical migration can take place, resulting in the formation of tin 'dendrites' which march across the surface of the assembly and can cause shorting. By stopping moisture from reaching the surface, dendritic growth can be prevented, particularly useful from the point of view of lead-free solders requiring more 'active' fluxes.

Any coating material will help in the strength enhancement of solder joints. However, with some of the newer semiconductor devices such as ball grid arrays (BGAs) the solder joints are tucked away underneath the component in extremely narrow gaps.

Most conventional liquid coatings have little chance of penetrating underneath such components. However, because it is applied from a vapour, Parylene coats into minute cracks and crevices and easily penetrates under these components. Independent tests carried out by the Institute of Production Engineering Research in Sweden concluded, 'One coating, Parylene, has been outstanding in the test. It gave excellent protection against corrosion and crack formation in solder joints:

Parylene doesn't stop there. It has been used successfully for the uniform encapsulation of millions of hygroscopic particles less than 1µm in diameter, to coat the inside of cylinders 25mm to 1.5m in length, and to coat a sealed electronic assembly through a 1.6mm orifice in the enclosure. It has been applied particularly as a means of preserving delicate artefacts including the remains of native American burial shrouds in Florida and 40 million year old botanical specimens found in the Canadian Arctic, as well as stabilising and preserving items salvaged from the Titanic and fire charred books from the Soviet Academy of Science in St Petersburg.

Parylene is being used for protecting sophisticated electronic circuits against the corrosive effects of chemical warfare decontamination agents and for coating cardiac pacemakers for insulation and biochemical resistance. In the States it is even being used to ensure that bridal bouquets remain the same indefinitely, in fact if keeping delicate objects intact and in pristine condition is the aim, Parylene seems increasingly to be the answer.

Primary author: Russ Wood

Source: Materials World, vol. 8, pp.30-32, June 2000.

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