# Parylene as conformal coating for PCBs

New to conformal coating and would like some recommendations.

From what I've read, it seems like parylene would be a good bet since it is chemically stable, makes an excellent barrier material, has excellent thermal endurance, as well as excellent mechanical properties & high tensile strength. Does anyone personally have any experience with using parylene coating?

• It depends on your requirements for moisture absorption rate and dielectric thickness effects on RF, high-speed logic load current or slew rate reduction or crosstalk. What is your environmental spec and design limit for load pF? – Tony Stewart Sunnyskyguy EE75 Sep 14 '18 at 2:08
• Parylene has much greater costs involved so it must be done in high volume to capture the benefits. – Tony Stewart Sunnyskyguy EE75 Sep 14 '18 at 2:19
• Note that a parylene coated board is, to all practical purpose, not reworkable or repairable. Repair is by board replacement. There are numerous other issues with parylene, such as not being able to put a board through the process with connectors soldered in many cases. – Peter Smith Sep 14 '18 at 9:11

There are many choices with tradeoffs that depend on your requirements for dielectric thickness (pF), protection from the environmental stress, reworkability, cost.

• AR - Acrylic - easiest to use, but prone to chemical solvent
• ER - Epoxy - highest protection, thermal or UV cure, hardest to repair
• UR - Urethane - polyurethane 1 or 2 part, UV cure excellent protection
• SR - Silicone - best for a high temp to 200'C
• PR - Parylene - highest cost, excellent protection.
• can be slightly flakey if underprocessed ( anecdotal inside HDA's )

Ref info: Diamond-MT

## Acrylic Resin (Type AR):

Acrylic conformal coatings are easily applied. They dry to the touch at room temperature in minutes, have desirable electrical and physical properties and are fungus resistant. They have long pot life and low or no exothermic during cure, which prevents damage to heat sensitive components. Also, they do not shrink. The main disadvantage is solvent sensitivity, but this also makes them easier to repair.

## Epoxy (Type ER):

Epoxy systems are usually available as “two-component” compounds. These rugged conformal coatings provide good humidity resistance and high abrasion and chemical resistance. They are, however, virtually impossible to remove chemically for rework because any stripper that will attack the coating also dissolves epoxy-coated or epoxy-potted components and the epoxy-glass printed circuit board itself. The only effective way to repair a board or replace a component is to burn through the epoxy coating with a knife or soldering iron.

## Polyurethane (Type UR):

Polyurethane conformal coatings are available as single component, two components, UV curable, and water borne systems. As a group, all provide excellent humidity and chemical resistance plus outstanding dielectric properties for extended periods.

## Silicone Type (SR):

Silicone conformal coatings are particularly useful for high temperature service, up to about 200° C. They provide high humidity and corrosion resistance along with good thermal endurance, making them desirable for PWA’s that contain high heat dissipating components such as power resistors. Silicone coatings are susceptible to abrasion (low cohesive strength) and have high coefficients of thermal expansion.

Performed per IPC-7711 and IPC-7721 Specifications.

## Parylene Type:

Parylene is a transparent polymer conformal coating that is deposited from a gas phase in a vacuum. These polymers are polycrystalline and linear in nature, possess superior barrier properties, have extreme chemical inertness, and because of the deposition process can be applied uniformly to virtually any surface and shape

## Surface cleanliness can affect conductivity

Plating chemistries,
Flux activators,
Perspiration ,
Ionic surfactants.


Parylene is great, maybe. It's applied using a CVD process so your workpiece has to be able to fit in the vendor's equipment. That's never been a problem for me, and probably won't be for you.

There are several different kinds of Parylene which your vendor can help you select. In my case I wanted one to keep water off of a hybrid ceramic circuit card. The coated device was great - it was coated all around in a single continuous layer - but the Parylene coating actually didn't bond at all to the ceramic. I was able to cut a slice down the middle and peel it off.

That would be okay in most applications. There was a small wiring harness coming off the board, and the wire was completely coated too, which was desired. In my case the finished assembly was to be potted, as well, and the Parylene coating bonded well to the potting (I work on underwater equipment and this is a major concern for us).

For less intensive requirements, one of the various Humiseal products may suit your needs. I can't remember which one I've used, but it's sprayed on and that can be done easily in your facility (assuming you have a suitable area for spraying volatiles). Because of the application process, the resulting coating is much less uniform, but this may be sufficient. The results I've had with such applications have been... sticky, compared to Parylene, which I liken to one of the nicest (and most expensive) approaches.

You could also pot the whole thing. There are tons of potting epoxies available. Ellsworth seems to own everything now, including Humiseal, but also tons of decent epoxy vendors. Their applications engineers (or sales guys, whoever answers when I call) are pretty reliable for standard applications (i.e. if you're not doing something too weird).

• I'll be looking for a spray on coating for homemade boards, so this is quite useful, thank you. I'll be photoresisting double sided board, sunlight curing under an oil paper or transparent sheet mask, and then dipping in homemade acid, and then I want to protect the contacts and coat the board with something. Is there one you would recommend for surface finish or ease of use if I don't need particularly high moisture resistance? When you mention that humiseal is sticky do you mean it remains sticky when dry? – K H Sep 14 '18 at 2:58
• When I was working on that Humiseal-coated project it had been very recently applied. It was dry but sticky. Maybe it gets better after a while, but even after a few days it still had a strange feeling to it. That was probably an acrylic one-part because things needed to be cheap. Call up Ellsworth, their phone guys are usually pretty good. Or you could try their product selector. Otherwise urethane-based coatings are usually robust, sprayable and might even look okay (my urethanes are thick potting and, though transparent, are hard to see through). – Jim Sep 14 '18 at 3:38

Parylene is the stuff that is vacuum deposited, isn't it? The material is sublimated from large chunks and sucked into another chamber where the PCB is where it is deposited.

We took our PCBs to a specialty house to have it applied.

Type UR conformal coat is much easier to work with. Can get it as a liquid or as a spray. Last time I checked Mouser.com carried it.