Parylene vs Liquid Conformal Coating: How to Choose

Conformal coating services › Parylene vs conformal coating

Choose parylene when you need a very thin, even coating that reaches into tight gaps, and choose a liquid conformal coating when you need easy rework, simple masking or high-volume application. The thickness ranges published for each type show the size of the difference: parylene (type XY) is applied at roughly 10 to 50 microns, while acrylic, epoxy and urethane liquid coatings sit at roughly 30 to 130 microns and silicone at 50 to 210 microns. This guide sets out what each coating does well, where each one causes problems, and a quick way to decide for your assembly.

Key Takeaways

• Parylene is deposited from vapour, so it coats edges, gaps and the undersides of parts evenly. Liquid coatings are sprayed, dipped or brushed, so coverage depends more on the method and the operator.
• Parylene is much harder to remove for rework. It resists solvents, so removal usually means laser ablation or micro-abrasion.
• Acrylic is the easiest liquid coating to rework, but it is weaker against strong solvents and one widely used acrylic is rated to 125°C continuous.
• Decide on the operating environment, the need for rework and your specification first. Cost per board comes after those.

What is the difference between parylene and liquid conformal coating?

Both protect circuit boards and components from moisture, contamination and corrosion. The difference is in how they are applied, and that drives almost everything else.

Liquid conformal coatings are resins applied wet and then cured or dried. IPC standards group them by chemistry: acrylic (AR), epoxy (ER), silicone (SR) and polyurethane (UR). They are applied by spray, dip, brush or robotic dispensing, and the right method depends on volume, how complex the masking is and the skill of the operator.

Parylene (type XY) is applied by chemical vapour deposition. The coating forms directly on every exposed surface at room temperature, with no liquid phase, so there is nothing to run, pool or bridge between leads. That is why parylene can be applied so thin and still give even coverage.

How thick is each coating?

Coating thickness requirements are set by IPC-A-610 according to the coating type. The ranges below are the published IPC-A-610 figures as summarised by Specialty Coating Systems, a coating supplier. Check the current revision of the standard and your own drawing before relying on them.

Thinner is not automatically better. Match the coating to the environment and your specification, not to the number.

Which coating should you choose?

The table below is our suggestion for a first decision. It is a starting point for a conversation, not a substitute for your specification or your customer's requirements.

When is parylene the wrong choice?

Parylene is not the answer to every coating question. Three situations count against it.

Rework is expected. Parylene is insoluble in common chemicals, so it cannot be wiped off with a solvent. Specialty Coating Systems notes that repair usually needs thermal or mechanical removal, such as laser ablation or micro-abrasion. If field repair is part of the product's life, a liquid acrylic is far easier to live with. IPC-7711/7721 covers procedures for coating removal and rework.

Masking is complex. Because vapour reaches every exposed surface, anything that must stay uncoated, such as connectors, test points or programming headers, has to be fully sealed before coating. Our suggestion is to agree a masking drawing before the first batch, whichever coating you choose.

The board just needs basic protection. For many commercial assemblies, a liquid coating applied to IPC-A-610 meets the requirement with a simpler process.

When is a liquid coating the wrong choice?

Liquid coatings can struggle where geometry or environment is demanding. Coverage depends on the method and the operator, so thin areas on sharp edges, pooling and shadowing around tall or dense components are the issues to check for at inspection. Acrylics in particular are limited by temperature and solvent resistance: Specialty Coating Systems gives 125°C as the maximum continuous operating temperature for one of the most widely used acrylic coatings, and notes that acrylics may not protect against stronger solvents.

What else affects the decision?

• Your specification. Many drawings name the coating type or an IPC-CC-830 qualified material. IPC-CC-830 sets qualification and performance requirements for coating materials and replaced the military specification MIL-I-46058.
• Adhesion. Some materials are hard to coat. Plasma cleaning before coating can help both parylene and liquid coatings adhere.
• The order of operations. Programme and test devices before coating, and mask programming points if they will be needed later. Our device programming team can plan this with the coating step.
• Tin whiskers. Coating can slow whisker growth but does not remove the cause. For pure tin finishes, see our tin whisker mitigation page.

How Systemation Euro can help

Systemation Euro offers both liquid conformal coating and parylene coating, with cleaning, masking, de-masking, inspection and a certificate of conformity if required. If you are not sure which coating your assembly needs, send us the drawing or specification and the operating environment through our conformal coating services page and we will advise.

Frequently Asked Questions

Is parylene better than conformal coating?

Parylene is a type of conformal coating, listed as type XY. It gives thinner, more even coverage than liquid coatings, but it is harder to rework. Whether it is better depends on the environment, the geometry of the assembly and whether rework is expected.

How thick is parylene compared with acrylic coating?

Published IPC-A-610 ranges put parylene at roughly 10 to 50 microns and acrylic at roughly 30 to 130 microns. Specialty Coating Systems says parylene is usually applied between about 13 and 51 microns, and acrylic at a dry film thickness of about 51 to 127 microns.

Can parylene be removed for rework?

Yes, but not with solvents. Parylene is insoluble in common chemicals, so removal usually means laser ablation or micro-abrasion. If rework is likely, an acrylic liquid coating is easier to remove.

Which conformal coating is easiest to rework?

Acrylic. It can be removed with relatively mild solvents such as isopropyl alcohol or xylene, which is also why it gives less protection against stronger solvents.

Do I need plasma cleaning before parylene?

Not always. Plasma treatment helps on materials that coatings struggle to adhere to. Tell us the materials and we will advise whether it is worth adding.

Sources: Specialty Coating Systems, Conformal Coating Specifications (IPC-A-610 thickness ranges, IPC-CC-830, IPC-7711/7721); Specialty Coating Systems, Comparing Parylene and Acrylic Conformal Coatings (application, rework, 125°C acrylic rating). Supplier sources; check the current revision of each standard for exact requirements.