Parylene FAQs

 


Why use Parylene conformal coating?

The Parylene conformal coating process is a specialised vapour deposition application process using specialist systems differing significantly to all of the other liquid conformal coatings available on the market.

This difference leads to many unique advantages:

  • Parylene coatings are completely conformal to the surface of the Printed Circuit Board (PCB) or product. This means that the coating has a uniform thickness and is pinhole free. As a result, component configurations with sharp edges, points, flat surfaces, crevices or exposed internal surfaces are coated uniformly without voids.
  • Parylene coating provides an excellent barrier that exhibits a very low permeability to moisture and gases. This means that products coating in parylene generally are more “waterproof” than the same products coated in a liquid conformal coating.
  • Parylene is unaffected by solvents, has low bulk permeability and is hydrophobic. Coatings easily pass a 100hr salt-spray test.
  • Parylene has excellent electrical properties. These include low dielectric constant and loss with good high-frequency properties, good dielectric strength, and high bulk and surface resistance.
  • Parylene is transparent and can be used to coat optical elements. No initiators or catalysts are involved in the polymerization so the coating is very pure and free from trace ionic impurities.
  • Parylene is unique in being created directly on the surface at room temperature. Room temperature formation means the conformal coating and the parts are effectively stress-free.
  • Parylene coating completely penetrates spaces as narrow 0.01mm (10 microns).
  • Parylene is chemically and biologically inert and stable and make excellent barrier material.
  • Parylene has good thermal endurance: Parylene C performs in air without significant loss of physical properties for 10 years at 80°C and in the absence of oxygen to temperatures in excess of 200°C.
  • FDA approval of parylene-coated devices is well-documented. The coatings comply with USP Class VI Plastics requirements and are MIL-I-46058C / IPC-CC-830B listed.

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How does the parylene application process work?

Parylene is applied through a specialised vapour deposition process at ambient temperature.

Parylene polymer deposition occurs at a molecular level, where the coating literally grows one molecule at a time on the substrate surface, assuring entirely conformal and uniform layers of parylene conformal coating are applied.

The process begins with the raw, granular material called the dimer which is heated under vacuum.

The dimer is then reduced to a gaseous state in the vaporising chamber.

The vapour is next drawn into the furnace and heated to very high temperatures (pyrolised) to allow for sublimation and the splitting of the molecule to a monomer.

The gas is finally drawn into the room temperature deposition chamber, where the monomer gas deposits on all surfaces as a thin, transparent polymer film and the result is a parylene conformal coating uniformly deposited on the product.

Because parylene is applied as a gas, the conformal coating easily penetrates everywhere on components, providing complete and uniform coverage. Literally a conformal coating.

While parylene coatings can range in thickness from hundreds of angstroms to several mils, a typical thickness is in the microns range.

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What type of equipment do you need for parylene application?

Parylene is applied to products like printed circuit boards through a specialised vapour deposition system.

The process comprises of three main sections: the sublimation chamber which operates at >100C, the pyrolysis section which operates are >500C the polymer deposition chamber which operates at room temperature.

Parylene polymer deposition occurs at a molecular level, where the coating literally grows one molecule at a time on the substrate surface, assuring entirely conformal and uniform layers of parylene conformal coating are applied.

Parylene Conformal Coating Process

  1. The process begins with the raw, granular material called the dimer which is heated under vacuum.
  2. The dimer is then reduced to a gaseous state in the vaporising chamber.
  3. The vapour is next drawn into the furnace and heated to very high temperatures (pyrolised) to allow for sublimation and the splitting of the molecule to a monomer.
  4. The gas is finally drawn into the room temperature deposition chamber, where the monomer gas deposits on all surfaces as a thin, transparent polymer film.
  5. The result is a parylene conformal coating uniformly deposited on the product.

The parylene coating process is controlled via a programmable logic controller. Each deposition system features an operator’s control panel that displays critical, factory pre-programmed process parameters. Process settings are accessible and can be changed by qualified personnel when necessary.

All machines feature closed-loop monomer pressure control, ensuring deposition of the polymer film at a precise rate. All operating temperatures and pressures are continuously monitored and any deviation from the acceptable limits results in audible and visual alarms.

Automatic process shutdown is initiated following process fault conditions of sufficient duration.

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How do you remove parylene conformal coating?

Parylene conformal coating removal can easily be achieved and can be broken down into three categories.

These are:

  • Thermal
  • Mechanical
  • Abrasion

Standard stripping techniques used in the area of liquid conformal coatings such as using liquid stripping fluids tend to be unsuccessful due to the inert nature of the parylene.

1) Thermal

The thermal parylene coating removal technique (including using a soldering iron to burn through the conformal coating) is the least recommended technique of coating removal.

Most conformal coatings require a very high temperature and/or long exposure times. This, in turn, can cause discoloration, leave residues, and adversely effect solders and/or other materials used in the construction of the board from boards.

Also, temperature-sensitive components may be damaged. Extreme caution must be taken when burning through conformal coating because some coatings emit very toxic vapors that are hazardous to the people doing the stripping and those around them.

2) Mechanical

Mechanical removal techniques include cutting, picking, sanding or scraping the area of parylene coating to be removed.

However, most types of parylene coatings are very tough to remove using this method making the probability of damage to the board very high.

3) Abrasion

The micro abrasive blasting technique offers a fast, cost-effective, easy to control and environmentally friendly non-solvent based method to remove conformal coatings.

The system can remove parylene conformal coatings from a single test node, an axial leaded component, a through-hole integrated circuit (IC), a surface mount component (SMC) or an entire printed circuit board (PCB).In the micro abrasive blasting process, a precise mixture of dry air or an inert gas and an abrasive media is propelled through a tiny nozzle attached to a stylus which is either handheld or mounted on an automated system.

This allows the mixture to be pinpointed at the target area of the parylene coating to be removed.

A vacuum system continuously removes the used materials and channels them through a filtration system for disposal.

The process is conducted within an enclosed anti-static chamber and features grounding devices to dissipate electrostatic potential.

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What are the common applications for parylene conformal coating?

Parylene is used in a huge variety of areas where critical coating application is a must have.

Areas where parylene is regularly applied include:

  • Printed circuit boards
  • Needles
  • Cardiac-Assist devices
  • Magnets
  • Stents
  • Motors
  • Catheters
  • Elasomesteric keypads
  • LED assemblies
  • Mandrels

Parylene film coatings are free of fillers, stabilizers, solvents, catalysts, and plasticisers.

Parylene N is a carbon-hydrogen molecule and parylene C is a carbon hydrogen molecule with a chlorine atom on the benzene ring.

Parylene coatings can be applied to most materials that are vacuum stable.

It has been successfully applied to paper, plastics, and metals.

Parylene provides a moisture and chemical barrier and is highly resistant to the damaging effects of the body. Parylene offers dry-film lubricity, with static and coefficients of friction near those of Teflon.

It’s most common applications are printed circuit boards, nitinol, precious metals, needles, and rubber products.

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What are tin whiskers?

Modern printed circuit boards are more environmentally friendly – they contain no lead. The downside is that lead prevented the growth and proliferation of tin whiskers and dendrites. In place of lead, metals such as tin, silver, cadmium and zinc were used.

Whiskers are thin, whisker-like filaments that can grow and ultimately bridge between 2 surfaces. As they are conductive, their presence can cause serious damage or failure in a circuit board. Dendrites are also damaging, but different. They appear as fern-like patterns.

The presence of whiskers/dendrites, can be devastating.

Particularly on:

  • Medical implants (pacemakers)
  • Aviation, Military Defense products
  • Aerospace electronics
  • Automotive circuits
  • Many consumer electronics

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How can Parylene help prevent tin whiskers?

Tin whisker mitigation is a field of research in itself. A great deal of work remains to be done.

What we do know, however, is that thorough board cleaning is helpful.

Some other suggestions include:

  • Minimize the use of pure tin or zinc
  • Use of physical barriers
  • Increase spacing
  • Conformal coat

Parylene coating is still one of the main “go-to” conformal coatings for tin whisker mitigation.

It offers good electric insulation and can penetrate all components of even the most densely populated PCB. In addition, Parylene does not affect the overall functionality of the electronic assembly.

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