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