A Thin Film That Decides Whether the Board Survives

Conformal coating is a polymer film, typically 0.001 to 0.005 in (25 to 130 um) thick, applied over a populated board. It follows the contours of the components, hence the name, and keeps moisture, condensation, dust, and airborne contaminants off the copper.

The failure it prevents is specific. Moisture plus surface contamination plus a voltage gradient produces electrochemical migration: metal ions move between adjacent conductors and grow a dendrite until the gap shorts. Flux residue accelerates it, as does the fine pitch on modern packages, where 0.008 in (0.2 mm) between pads is a short distance to bridge. The result is a board that works for months and then fails intermittently in the field.

When a Board Genuinely Needs It

Coating is not free and not always right. It earns its place when one or more of these applies:

  • Condensation cycling. Outdoor equipment, unheated warehouses, and vehicles. Cycling is worse than steady humidity because it deposits liquid water on the surface repeatedly.
  • High humidity operation, above roughly 85 percent RH for sustained periods: greenhouses, marine environments, laundry and food equipment.
  • Chemical, salt, or dust exposure. Coastal installations, agricultural chemicals, sulfur-bearing industrial atmospheres that corrode silver and copper, and conductive dust from grinding or carbon that bridges pads directly.
  • High voltage on the board. Coating raises the effective dielectric strength across a gap and can let you keep clearances that would otherwise need more board area.

Conversely, a board inside a climate-controlled office product or a handheld carried indoors usually does not need it. Coating those boards adds cost, complicates rework, and buys very little.

Coating the Board Is Not the Same as Sealing the Housing

These two get treated as interchangeable and they solve different problems.

A sealed enclosure keeps bulk water and dust out of the whole assembly. It does nothing about condensation forming inside from air that was sealed in, and a sealed box in a temperature-cycling environment is an efficient condensation chamber. It also fails suddenly if one gasket is misassembled. Verifying a real seal is its own discipline, costed under IP ratings.

Conformal coating protects the circuitry itself regardless of what the housing does. It tolerates condensation, degrades gradually rather than catastrophically, and protects during handling and storage before the housing goes on.

The right answer for harsh environments is often both: a sealed housing with a breathable vent membrane, plus a coated board as the second line of defense. For severe cases, potting the assembly in a filled resin replaces both, at the cost of weight, thermal behavior, and any possibility of repair.

Material Families and What Separates Them

Acrylic (AR). The default. Cheap, fast-drying, and removable with common solvents, which makes rework simple. Good moisture resistance, poor abrasion and solvent resistance, temperature limited to roughly 250 F (120 C). Fluoresces under UV, so coverage is easy to inspect.

Polyurethane (UR). Better chemical and abrasion resistance than acrylic. Harder to rework, since removal needs aggressive solvents or abrasion. The choice where fuels, oils, or solvents are present.

Silicone (SR). The wide temperature performer, from below -60 F to above 390 F (-51 C to 199 C), and it stays flexible, which suits large thermal expansion mismatch or vibration. Softer and easily damaged, and silicone contamination is a real concern in facilities that also paint or bond.

Parylene (XY). Deposited from vapor in a vacuum chamber, producing a pinhole-free film that reaches under low-standoff components where liquid coatings cannot go. The best barrier by a wide margin and the most expensive: batch processing at a specialized vendor, and no practical rework. Used on implantable medical electronics and high-reliability aerospace boards. Epoxy (ER) sits at the other extreme, very hard and essentially unremovable, and rigid enough to crack components under thermal stress.

Whatever the family, specify to IPC-CC-830 or equivalent and state the thickness range and the keep-clear areas. "Conformal coat the board" on a drawing gets you whatever the assembler feels like doing.

What It Does to the Production Process

Coating adds a real step, not a spray booth afterthought.

The board must be genuinely clean first. Flux residue trapped under coating is worse than no coating, because it is now sealed against the copper with moisture available to it. That means either a no-clean process validated for coating or an aqueous wash and dry cycle, and it is the step most often skipped.

Then everything that must stay uncoated has to be masked: connectors, test points, mating surfaces, heatsink pads, switches, and any part with a vent hole. Masking is manual labor and the dominant cost driver in low volume. Group connectors along one edge so a single boot covers them, and remember that masked test points are what keep the production test fixture usable.

Application methods scale differently. Manual brushing is fine for tens of units and inconsistent beyond that. Aerosol spraying is quick and wastes material. Selective robotic dispensing is the volume answer, applying coating only where specified with little masking, and it typically pays for itself in the low thousands of units per year.

Budget roughly $2 to $10 per board in low volume, mostly labor, dropping toward $0.50 to $2 with selective dispensing at volume. Parylene runs far higher, commonly $15 to $60 per board. Add cure time to the line: minutes for UV-cure materials, hours at elevated temperature for others. Inspect coverage under UV light and write an acceptance standard into the process documentation, since thin spots at component edges and bubbles over vias are the common defects.

What Coating Will Not Fix

Coating is a moisture barrier, not a general remedy, and expecting more from it causes disappointment.

It does not make a board waterproof. A coated board immersed in water fails at the connectors and at any bare pad, so it is no substitute for an enclosure in that duty.

It does not help with heat. Coating adds a slight thermal barrier if anything, so the thermal design still has to stand on its own as described in thermal management in electronic products.

It does not fix a bad layout. Insufficient creepage between high-voltage nets, missing return paths, or components too close to a board edge remain problems under a film. Coating buys margin on clearance; it does not create clearance.

It makes rework harder and sometimes impossible, which matters if your service model involves board repair. And it will not compensate for a design that has never been through a real reliability testing program: humidity and thermal cycling on coated and uncoated samples is how you prove the coating is doing something.

Decide Whether Your Board Needs Coating

Projects House specifies board protection for products going into harsh environments: whether to coat, which material, what to mask, how to apply it at your volume, and how to verify it with humidity and thermal cycling. Send your board files and operating environment through our contact form.