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Types of Conformal Coating for PCB Protection

Oct 19, 2020

types of conformal coatingWith the rise of IoT and technology in general, electronics are featured in myriad environments. This means that sensitive electronic connections are potentially exposed to hazards like extreme temperatures, moisture, and dust that can harm components and disrupt the integrity of the assembly itself.

One preventative measure you can take is the use of PCB conformal coating during the production process. This procedure applies a thin transparent film to a circuit board, which contours to its shape and protects its components.

If your assembly will potentially face exposure to adverse conditions, you should consider whether conformal coating is right for your design. If so, consider which types of conformal coating make the most sense and with which method should you apply it.

With that in mind, let’s go over some of the types of conformal coating at your disposal

Types of Conformal Coating

Popular types of PCB conformal coating include:

  • Brush
  • Spray
  • Dipping
  • Selective coating

Brush Method

A simple application method, this is best for low-volume production, rework, and repair.

Brush coating is done by hand and tends to be thicker and less cosmetically appealing. This option is best for those without the tools to utilize other options or when working on a few boards at a time.

Spray Method

This method involves using an aerosol spray to apply the coating. Also better for lower-volume production, spraying can be time-consuming because all areas that don’t require coating need to be masked.

This method provides a superior surface finish and is cost-effective, but doesn’t offer deep penetration of the board.

Dipping Method

“Dipping” involves submerging a board in coating solution, then withdrawing it.

Good for high-volume production, this method is fast, accurate, and completely penetrates the board, coating the entirety of the assembly. However, masking must be perfect to avoid leakage, making many boards unsuitable due to design. Only boards that accept coating on both sides can be subject to this method.

Selective Coating Method

This strategy uses automated robotic spray nozzles to apply conformal coating to specific areas of the assembly.

Another high-volume method, selective coating is fast and accurate and applies the coating directly to the areas of the board where it is required, eliminating the need for masking.

You must design your circuit board to be compatible with selective coating.

Types of Conformal Coating

Acrylic Resin (AR)

Pros Cons
Easy rework Difficult to maintain viscosity
Easy drying process Flammable
Good humidity resistance High probability of reversion under temperature and humidity stress
High fluorescence level
Ease of viscosity adjustment

Epoxy

Pros Cons
Useful to around 150fC [302fF] Higher chloride contamination potential
High chemical and abrasion resistance Process intensive, difficult to maintain viscosity
CTE closer to epoxy PCB substrate Complex mix ratios
Good dielectric properties Potential for high stress during temperature cycling conditions
Good humidity resistance Difficult to rework and remove
High probability of reversion under temperature and humidity stress
Lacks flexibility

 

Urethane (Polyurethane) Resin (UR)

Pros Cons
Good dielectric properties Difficult to maintain viscosity
Good moisture resistance Flammable
Solvent-resistant High probability of revision under temperature and humidity stress
Less reversion potential Health and safety concerns
Abrasion-resistant Potential for high stress during cycling conditions

 

Silicone Resin (SR)

Pros Cons
High dielectric strength Short pot life
Fair moisture and abrasion resistance Does not protect against solvents or solvent vapors
Good moisture, humidity, and UV/sunlight resistance If proper housekeeping is not followed, there is a potential for cross contamination
Low surface energy to enable effective penetration under components Requires humidity (minimum 20% RH) to cure and only intermittent solvent resistance
Flexible, provides dampening and impact protection
Stable over wide temperature range (in general, -40fC to 200fC) [104fF to 392fF]

 

Poly-Para-Xylelene C, D, N

Pros Cons
Minimal added mass and low outgassing Cannot be doped
Low environmental impact process Masking required for no-coat areas
Biocompatibility allows use in medical applications Parts are processed in batches in a vacuum chamber, not an in-line process
Excellent uniformity regardless of part geometry, no pinholes, fillets, or bridging Coating removal and rework generally requires specific equipment, abrasion/microblasting most common technique
Chemical intertness/barrier properties -- insoluble in organic solvents, acids, or bases with very low permeability rates Limited UV resistance and operating temperature limit, around 120fC [248fF] in the presence of oxygen

 

Amorphous Fluoropolymer

Pros Cons
Low dielectric constant Requires special liquids for polymer swelling
High glass temperature Limited solubility which limits film thickness
Low surface energy May require glass temperature annealing
Low water sorption Requires special surface treatment for greatest adhesion
Good solvent, oil, and common acid resistance Poor resistance to some acids and alkalines

 

Fluorinated Poly-Para-Xylelene

Pros Cons
Low dielectric constant, 2.28 Parts are processed in batches in a vacuum chamber, not an in-line process
Low environmental impact process Masking required for no-coat areas
High temperature stability (450fC) [842fF], and increased UV stability The coating is deposited at a rate slower than the conventional poly-para-xylenes
Chemical intertness/barrier properties -- insoluble in organic solvents, acids, or bases with very low permeability rates Coating removal and rework general requires specific equipment, abrasion/microblasing most common technique
Excellent uniformity regardless of part geometry, no pinholes, fillets, or bridging Requires special deposition equipment different than that for the C, D, and N poly-para-xylene varieties

 

Finding the Right Coating for Your PCBs

The right conformal coating will vary depending on the application for your PCBs. Deciding which coating can use can be a challenge, but when you work with an electronic contract manufacturer, you can find the best option for coating your board. 

Don't forget to check out our free guide to SMD resistor and capacitor sizes!

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Editor's Note: This article was first published in 2019 and has since been updated. 

Matric

Written by Matric

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