How Conformal Coatings Protect PCBs From Moisture and Salt

Sprayed acrylic, silicone, and urethane conformal coatings protect assembled circuit boards in harsh environments by creating a microscopic, insulating barrier that physically isolates conductive traces and component leads from moisture and corrosive salt fog. This article provides a comprehensive overview of the failure mechanisms caused by humidity and salt, explains how conformal spray coatings prevent electrical shorts and corrosion, and breaks down the distinct defensive properties of acrylic, silicone, and urethane materials.

The Threat of Moisture and Salt Fog

Harsh-environment electronics face constant exposure to humidity, condensation, and marine air. Water alone can cause minor issues, but when combined with ionic contaminants like salt fog, it creates a highly conductive electrolyte solution on the surface of a printed circuit board (PCB).

Without protection, this conductive film bridges adjacent solder joints and traces, leading to immediate parasitic leakage currents and short circuits. Over a short period, an electrical bias combined with moisture and salt triggers electrochemical migration. This process grows conductive metal filaments—known as dendrites—across non-conductive spaces, resulting in catastrophic and permanent board failure. Furthermore, chloride ions in salt fog aggressively oxidize copper traces, solder joints, and component leads, physically eating away the circuitry.

How Conformal Coatings Shield the Circuitry

When sprayed onto an assembled PCB, a conformal coating flows around the complex topography of the components and cures into a thin polymeric film, typically between 25 and 250 microns thick. This film defeats moisture and salt fog through three primary mechanisms:

  1. Physical Isolation: The cured polymer is hydrophobic and non-conductive. It prevents water droplets and salt crystals from making direct physical contact with the metal surfaces of the board. Even if salt fog settles on top of the coating, the underlying circuitry remains electrically isolated.
  2. Surface Adhesion: A properly applied coating bonds tightly to the PCB substrate and component bodies. This tight molecular bond prevents moisture vapor that eventually permeates the film from condensing into liquid water pools at the board's surface. Without a liquid layer, ionic salts cannot dissolve to form a conductive bridge between electrical nodes.
  3. Dielectric Insulation: The coating boosts the dielectric strength between closely spaced conductors. This stops electrical arcing and corona discharge, which are much more likely to occur in damp, salt-laden air.

Acrylic Coatings: The Moisture Barrier

Acrylic conformal coatings (Type AR) are solvent-based or water-borne polymers that cure rapidly at room temperature via solvent evaporation.

Acrylics provide an exceptional physical barrier against humidity and salt spray. Because they have very low moisture absorption rates, they effectively block salt ions from reaching the circuit traces. They also maintain high dielectric strength in wet conditions. While acrylics are ideal for general moisture and salt fog protection, they are vulnerable to chemical solvents and have a narrower operating temperature range compared to other chemistries. Their primary advantage is easy reworkability; they can be dissolved with mild solvents for board repairs.

Silicone Coatings: Extreme Environment Defense

Silicone conformal coatings (Type SR) are flexible elastomers designed for environments that combine moisture and salt with extreme temperatures, often functioning reliably up to 200°C.

Interestingly, silicone has a higher water vapor transmission rate than acrylic or urethane, meaning moisture vapor passes through it easily. However, silicone successfully shields the PCB through superior surface adhesion and hydrophobicity. Because the silicone is chemically bonded to the board surface, the passing vapor cannot condense into a liquid film on the metal conductors. Liquid water droplets and heavy salt spray on the exterior simply bead up and roll off the flexible, rubbery surface. Its flexibility also absorbs mechanical stress, preventing the coating from cracking during rapid thermal cycling in marine or aerospace applications.

Urethane Coatings: Heavy-Duty Corrosion Resistance

Urethane (polyurethane) conformal coatings (Type UR) are available as single-component, moisture-cure, or two-component systems. They cure into a hard, durable shell.

Urethanes offer the most robust defense against prolonged exposure to salt fog and corrosive marine atmospheres. They possess very low moisture permeability, preventing both liquid water and water vapor from penetrating to the board level. In addition to blocking salt and humidity, urethanes are chemically cross-linked, making them impervious to harsh chemical solvents, fuels, and hydraulic fluids that often accompany salt fog in industrial, naval, and automotive environments. Because of this extreme durability and chemical resistance, urethane coatings are very difficult to remove, making PCB rework challenging.

Ensuring Complete Protection Through Application

The chemical properties of acrylic, silicone, and urethane only work if the sprayed application is flawless. Automated selective spray systems are typically used to ensure consistent film thickness.

If a board is not thoroughly cleaned before spraying, ionic residues trapped beneath the coating will draw in moisture through osmosis, leading to blistering (mealing) and internal corrosion. Furthermore, the spray process must eliminate pinholes, bubbles, and shadowed areas behind tall components. Sharp component leads have high surface tension that pulls liquid coating away from the edges as it cures; applying multiple thin mist coats ensures these sharp points are fully encapsulated against salt fog intrusion.