Conformal Coating Types: Acrylic, Silicone and Parylene

A conformal coating is a thin polymer film applied over a populated board to keep moisture, salt, dust and chemical vapour away from the conductors and joints. It is one of the cheapest reliability improvements available, and one of the easiest to get wrong, because the material has to suit the environment, the application method has to be controlled and the masking has to be planned before the design is finished.

What the Coating Actually Does

The film works by excluding the electrolyte that corrosion needs. Moisture on a dirty board creates a leakage path between conductors, and with bias applied that path drives electrochemical migration, which grows dendrites and eventually shorts the circuit. Removing the water from the surface removes the mechanism.

It also provides mechanical protection. A coating immobilises small components, raises the voltage at which surface tracking becomes a problem, and reduces the damage caused by handling and vibration. What it does not do is make a wet assembly dry, so the board must be clean and dry before the coating is applied.

Acrylic Coatings

Acrylic is the general purpose choice. It cures quickly, is easy to apply by spray or dip, and is the simplest material to rework because a solvent softens it locally. Moisture resistance is adequate for indoor equipment and for products that see occasional condensation.

Its weakness is temperature and chemical resistance. Acrylic softens at relatively modest temperatures and is attacked by some solvents, so it is a poor choice for a board that runs hot or is exposed to fuel, oil or aggressive cleaning agents.

Urethane and Silicone Coatings

Urethane coatings are tougher and more resistant to chemicals and abrasion than acrylic, and they hold up well against solvent exposure. Their cost is difficulty of rework, since they resist the solvents that make acrylic easy to remove, and the cured film is hard enough that repair usually means cutting it away.

Silicone coatings are chosen for temperature range and flexibility. They remain resilient from very cold to well above the working range of the other materials, which makes them the natural choice for automotive and aerospace hardware. They are also the most forgiving of thermal cycling, because the film flexes with the assembly instead of cracking, and the practice around this is described in conformal coating and board protection.

Parylene and Vapour Deposition

Parylene is applied by vapour deposition in a vacuum chamber rather than by spraying or dipping. The monomer deposits directly onto every exposed surface, including the undersides of components and the inside of gaps, producing a uniform, pin hole free film with no liquid flow and no edge build up.

The result is the best barrier available, and the process is the most expensive. It also requires the board to be masked carefully because the film gets everywhere, and removal for rework is done by abrasion or laser rather than by solvent, which limits how much repair is practical afterwards.

Application Methods

Spraying gives the most control over thickness and is suited to selective coating, where the machine applies material only to defined areas using a programmed path. Dipping gives a uniform film and high throughput but coats everything, so masking must be complete. Brushing by hand is used for repair and for small batches, and it is the least repeatable.

Whatever the method, the process parameters determine the result. Viscosity, spray pressure, dispense rate, dwell time and the number of passes all affect film thickness, and the thickness has to be measured on a coupon or on the board itself. A coating that is half the specified thickness provides a fraction of the expected protection.

Masking and Design Rules

Masking is where most production problems originate. Connectors that must mate, test points that must be probed, pressure sensors with a port, relays, batteries and anything with moving parts all have to be excluded. The masking is applied by tape, by boot or by dispensed gel, and each adds labour.

The design can reduce that labour. Keeping connectors on one edge, grouping test points together and leaving a defined keep out around parts that must stay clear all make the masking operation simpler and less error prone. Raising a part slightly off the board so the coating can flow underneath, or deliberately leaving a gap, also helps where a coating bridging two pads would change the circuit.

Curing and Inspection

Curing follows the material specification, and both time and temperature matter. A coating that is handled before it is fully cured can be displaced, and a thick coating can trap solvent underneath, which produces bubbles that break the barrier. Where the material cures by moisture, the ambient humidity becomes a process parameter.

Inspection is done by ultraviolet light where a fluorescent tracer is added to the material, which shows coverage clearly and reveals the areas where the film is thin or missing. Without the tracer, inspection relies on visual checks under white light, which are far less reliable on a board covered in components.

Choosing a Material

The decision follows the environment first. Indoor equipment with occasional condensation is served by acrylic; a product exposed to solvents or abrasion needs urethane; a board that swings through a wide temperature range needs silicone; and hardware where the barrier is critical and rework is rarely acceptable justifies parylene.

The second consideration is the repair strategy. If the product will be reworked during debugging or in the field, a material that can be removed locally saves a great deal of time. Choosing parylene for a prototype build, or urethane for a board that will be reworked, creates cost that the environment did not require.

Process Control and Documentation

Coating is a special process, meaning that the finished result cannot be fully verified by inspecting the board. Control therefore rests on the specification and the records: material batch, viscosity, application parameters, cure schedule, thickness measurement and inspection result.

Those records also make the process repeatable across shifts and sites. A coating operation that depends on one experienced operator is a risk, and the same reasoning that applies to any other special process, including the dispensing work described in potting, dispensing and adhesives, applies here.

FAQ

Does the board have to be cleaned before coating? Yes, and the cleaning is more important than the coating. Flux residue trapped under a film causes the corrosion the coating was meant to prevent, and ionic contamination also weakens adhesion.

Can a coated board be repaired? It depends on the material. Acrylic can be softened locally and touched up afterwards, while silicone is more difficult and parylene is effectively permanent. The repair plan should be part of the material choice.

How thick should the coating be? The material specification gives a range, commonly twenty five to a hundred and thirty micrometres depending on the type. Thinner films lose protection, and much thicker films risk cracking and trapped solvent, so the range should be measured rather than estimated. The joint quality underneath the film is fixed before the coating is applied, which is why the alloy and profile choices discussed in lead-free versus leaded solder still determine the reliability of a coated assembly.