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:
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Brush
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Spray
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Dipping
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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
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Ease of rework
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Ease of drying process
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Good humidity resistance
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High fluorescence level
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Ease of viscosity adjustment
Cons
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Difficult to maintain viscosity
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Flammable
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High probability of reversion under temperature and humidity stress
Epoxy
Pros
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Useful to around 150 fC [302fF]
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High chemical and abrasion resistance
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CTE closer to epoxy PCB substrate
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Good dielectric properties
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Good humidity resistance
Cons
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Higher chloride contamination potential
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Process-intensive, difficult to maintain viscosity
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Complex mix ratios
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Potential for high stress during temperature cycling conditions
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Difficult to rework
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High probability of reversion under temperature and humidity stress
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Difficult to remove
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Lacks flexibility
Urethane (Polyurethane) Resin (UR)
Pros
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Good dielectric properties
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Good moisture resistance
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Solvent-resistant
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Less reversion potential
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Abrasion-resistant
Cons
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Moisture affects cure rate and desired properties
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Long complete cure time (up to 30 days)
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Health and safety concerns
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Potential for high stress during temperature cycling conditions
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High probability of reversion under temperature and humidity stress
Silicone Resin (SR)
Pros
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Stable over wide temperature range (in general, -40ƒC to 200ƒC)[104ƒF to 392ƒF]
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Flexible, provides dampening and impact protection
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Good moisture, humidity, and UV/sunlight resistance
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High dielectric strength
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Fair moisture and abrasion resistance
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Low surface energy to enable effective penetration under components
Cons
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Short pot life
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Does not protect against solvents or solvent vapors
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If proper housekeeping is not followed, there is a potential for cross contamination
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Requires humidity (minimum 20% RH) to cure and only intermittent solvent resistance
Poly-Para-Xylelene C, D, N
Pros
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Excellent uniformity regardless of part geometry -- no pinholes, fillets, or bridging
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Chemical inertness/barrier properties -- insoluble in organic solvents, acids, or bases, with very low permeability rates
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Minimal added mass and low outgassing
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Biocompatibility allows uses in many medical applications
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Low environmental impact process
Cons
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Parts are processed by batches in a vacuum chamber, not an in-line process
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Masking required for no-coat areas
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Coating removal and rework generally requires specific equipment, abrasion/micro-blasting most common technique
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Limited UV resistance and operating temperature limit, around 120ƒC [248ƒF] in the presence of oxygen
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Cannot be doped
Amorphous Fluoropolymer
Pros
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Low dielectric constant
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High glass temperature
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Low surface energy
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Low water sorption
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Good solvent, oil and common acid resistance
Cons
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Requires special liquids for polymer swelling
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Limited solubility which limits film thickness
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May require glass temperature annealing
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Requires special surface treatment for greatest
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Adhesion
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Poor resistance to some acids and alkalines
Fluorinated Poly-Para-Xylelene
Pros
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Excellent uniformity regardless of part geometry ‘no pinholes, fillets, or bridging
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Chemical inertness/barrier properties ‘ insoluble in organic solvents, acids, or bases, with very low permeability rates
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High temperature stability (450ƒC [842ƒF]) and increased UV stability
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Low dielectric constant, 2.28
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Low environmental impact process
Cons
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Parts are processed by batches in a vacuum chamber, not an in-line process
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Masking required for no-coat areas
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Coating removal and rework generally requires specific equipment, abrasion/micro-blasting most common technique
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The coating is deposited at a rate slower than the conventional poly-para-xylelenes
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Requires special deposition equipment different than that for the C, D, and N poly-para-xylelene varieties
