What is Conformal Coating? Why do I need it?

Conformal coating is a protective polymeric film that is applied to the part and conforms to the topography. The primary purpose is to provide protection specific to the parts needs. Each coating type has unique chemistry which is tailored to specific protective properties. This means that a trade off usually has to be made when choosing a coating type. We haven’t discovered the super coating that works on all parts just yet, although Parylene is getting closer…we will get to that shortly.

The question I get the most is what type of coating should I apply to my parts? This question seems simple in theory, but there are many layers that need peeled back before you should pick a coating type. Some of the initial questions you should ask yourself before starting down this path are:

1. What am I trying to achieve with this coating?
a. Dielectric properties
b. Chemical resistance
c. Water resistance
d. Protection from dust or debris
e. Lubrication
f. Barrier Protection
g. Protection from vibration
h. Abrasion resistance

2. Do I need to meet certain requirements with the coating I choose?
a. Mil Spec
b. UL listed
c. Class VI Certified
d. ROHS compliant
e. AR70/AFR80/Navinst 3400.2
3. How quickly do I need my parts?
a. Cure time considerations
b. Thickness requirements
c. What method of coating

4. How big are my parts?
a. If I am choosing Parylene, will it fit in the coating chamber?
b. Will I need special fixturing to coat the boards?
c. How much coating will be required to achieve the desired thickness?
d. Will the cost be affected?

Once these questions have been asked and narrowed down the conformal coating selection may begin. This would also be a good time to call Para-Coat Technologies for specific questions that may need answered. We have a team of people dedicated to helping you get the answers you need.
Types of conformal Coating
There are five main categories of conformal coating.

1. Urethane resin (UR)
Can be 1-part or 2-part substances. Single component has a longer pot life but much longer cure times and is easier to apply while two-part may have a shorter pot life but have faster cure times. The addition of heat can speed up the curing process

Abrasion resistant Long Cure Times
Excellent humidity resistance Difficult to remove once fully cured
Great chemical resistance Short pot life
Solvent resistant Solder iron needed for spot repair

2. Acrylic resin (AR)
Typically, 1-part. These are probably the most common and easiest coating to remove and repair. Acrylics dry rapidly, reaching optimal properties in minutes rather than hours.

Ease of application Low abrasion resistance
Easily repairable Low chemical resistance
Faster Cure time Not great in extreme environments
Fungus resistant Low resistance to high temperature environments

3. Epoxy resin (ER)
Maybe the best protective properties of the liquid conformal coatings but the most challenging to work with. The coating tends to shrink during the curing process which can cause the need for additional material.

Heat resistance Very difficult to remove
UV resistance Shrinkage during curing process
Very good in extreme environments Localized repair or rework can be challenging
Terrific abrasion resistance

4. Silicone resin (SR)
This is the most difficult coating to remove and the coating least compatible with all of the other coating types. There are no solvents to remove silicone so it may only be removed by abrasion and this will leave silicone contamination on the board surface. Complete removal is virtually impossible so local repair is generally all that is attempted.

Good adhesion to most surfaces Difficult rework
Withstands prolonged high temp exposure Not compatible with other coatings
Useful for vibration dampening Can only be removed mechanically
Very good chemical/humidity/corrosion resistance

5. Parylene (XY)
Parylene is the only coating that not applied using traditional spray, brush or dip methods. It is applied using a Chemical Vapor Deposition Process with specialized equipment. The Polymer raw dimer material is heated up and vaporized into a monomer before entering the coating chamber. It is then deposited on the surface of the product as a polymer at the molecular level. This allows for a very thin coating (.5 um to 1 mil) but still maintains protective properties greater than or equal to other coatings at 2-7 mil thicknesses.

Very high dielectric strength Not ideal for direct UV exposure
No cure time required Need for specialized equipment
Optically clear Very difficult to remove/rework
Best solvent and temperature resistance
Can be applied to a wide range of materials

Parylene really is an amazing coating due to the many applications it can serve. It’s adhesion on so many different substrates (including metal, silicone, rubber, fabric, circuit cards etc.) make it truly unique. Parylene has some of the best protective qualities out of any of the other conformal coatings listed and does so at very thin coatings. Parylene is also class VI certified implantable, which makes it ideal for countless medical applications. Out of all of the coatings we offer, Parylene is the one that I recommend the most. It’s track record in the Electronic, Medical, Automotive and Aerospace industries are well documented and proven. When a problem arises, Parylene is almost always the first coating I look to for a solution.
All coatings offer some type of protection, but it is a matter of choosing the proper coating to best protect the parts in the environment that they will be placed into. Consider all of the factors listed above before choosing a conformal coating because picking the right material is critical to the longevity of the part.

Blog written by General Manager David Pribish
Contact him to discuss your application!

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