Best Conformal Coating for Moisture and Chemical Protection

Polymeric conformal coatings safeguard printed circuit boards (PCBs) from performance malfunction caused by contact with elements within their operational environment.  Included are:

• moisture presence within the PCB, altering its thermo-mechanical properties, and
• chemical incursion, which can quickly corrode PCB components.

More than these specific causes of PCB malfunction, conformal coatings need to provide an overlying measure of basic protection, ensuring the PCB works as designed.

Each coating material – liquid acrylic, epoxy, silicone or urethane, and vapor-applied parylene – possesses singular chemical/performance properties determining its value for moisture/chemical/basic assembly protection.

Moisture Protection

Disrupting a PCB’s thermo-mechanical properties, excess moisture causes:

• corrosion,
• decreased glass transition temperature (Tg),
• differential swelling/hygroscopic stress,
• diminished interfacial adhesion strength, and
• metal migration/electrical-shorting.

Conformal protection controls moisture-originated performance issues; success-level depends on the film-material used.

Liquid acrylic resin (AR) provides reliable PCB moisture protection/humidity resistance, with low Tg temperatures.  Minimal shrinkage during curing/operation ensures AR meets basic moisture-barrier requirements for fully-covering exposed PCB surfaces.  For all coatings, eliminating exposed leads/coating-voids like bubbles/cracks-fissures/de-wetted areas is imperative; these become sites of corrosion within the film and underneath, on the PCB’s surface.  While AR meets these important performance standards, lower resistance to abrasives/chemicals/solvents interferes with moisture protection for assemblies working in harsh operational environments.

Liquid epoxy’s (ER) hard conformal coatings provide useful hydrophilic polar protection, but water migration remains possible, stimulating corrosion/lessened performance.  Excessive moisture increases dielectric constant (Dk) and dissipation factor (Df), changing circuit switching-speed and delaminating coating.  Swelling underneath the coating can cause epoxy-peeling, leaving the assembly unprotected.

Adhering well to a wide range of substrates, wet-applied Silicone (SR) is hydrophobic, with good moisture control.  Despite superior humidity resistance, moisture-penetration can be high; SR is susceptible to moisture-concentrations/corrosion when exposed to air pollutants/salt-fog carrying chloride/sodium/sulfur dioxide ions.

Liquid urethane’s (UR) resistance to humidity and liquid chemicals is generally high, for many PCB conditions.

CVD-deposited parylene (XY) films provide the thinnest effective coating available, with excellent substrate coverage.  The exceptional uniformity of pinhole-free XY prevents leakage; vaporous coatings seep deep into substrate surfaces during application, generating additional moisture protection.

Chemical Protection

Contact with harsh chemicals can corrode conformal coatings and components underneath.

Least resistant to chemical incursion, AR coatings easily dissolve in many organic solvents; while this is ideal for coating repair, chemical resistance is low and very selective.

Epoxy resins have good abrasive/chemical resistance; very durable, ER withstands the impact of most solvents; removal requires burning with a soldering iron.

Used primarily in high temperature environments, chemically-inert SR films offer PCBs good chemical protection; applied in thicker film-layers than other coatings, their chemical resistance can make them difficult to rework.

Urethane provides high chemical resistance, requiring higher-strength stripping agents for removal; these can corrode the PCB if not immediately cleaned after UR removal.  Soldering may also be necessary if chemical removal is ineffective.

Chemically inert parylene has outstanding chemical resistance, sustaining PCB function in harsh environments, characterized by atmospheric pollutants and aggressive solutions better than liquid coatings.

Basic Protection

Basic conformal protection dispels incidents of PCB current-leakage/mechanical stress caused by ongoing use, physical shock, temperature extremes, and vibration.  Equally important is excluding dirt/dust/fungus/salt sprays that interfere with assembly performance, while maintaining nonconductive/insulative/dielectric properties appropriate to operation.  Each coating has advantages/disadvantages dependent upon their chemical-physical properties/deposition method/re-workability/cost.

Table 1 compares coating properties among material types, using a Likert Scale, where 1 = lowest rating, and 5 = highest rating.

Type/Property	Acrylic	Epoxy	Urethane	Silicone	Parylene
Cost	Low	Moderate	Moderate	Moderate	High
Application Ease	5	3	4	2	2
Dry/cure time	5	5	3	3	5
Chemical resistance	1	3	4	4	5
Solvent resistance	2	3	5	3	5
Moisture resistance	4	4	5	4	5
Heat resistance	1	1	3	5	5
Dielectric properties	5	5	4	5	5
Comments	Represents 80% conformal coating market	Service temperature: up to 150°C	High chemical/ solvent resistance	Service temperature: 40°C to 200°C	Dielectrically superior/chemically. inert

 

Inexpensive, easily-applied AR is the most commonly-used conformal compound, with good moisture protection, but lower chemical resistance and abrasive/stress-relieving properties; AR has limited value for performance conditions characterized by higher operating temperatures/prolonged solvent-exposure/longer-term coating strength.

Exceptional surface hardness/service-durability/good dielectric properties/beneficial Tg-temperatures are ER’s basic performance benefits: drawbacks include coating-shrinkage during polymerization/lowered stress-resistance when subjected to thermal extremes/removal difficulty.

With good moisture/chemical protection and a workable temperature range, SR’s material properties require thicker liquid-application than other coatings.  Inert biologically and chemically, SR offers flexible, impact-dampening protection, and high dielectric strength.  However, limited solvent resistance interferes with silicone’s bonding-ability, promoting delamination.  Thick, rubbery films are unsuitable for tight clearance-tolerances/solder joints and MEMS/nano-technology.

Very resistant to abrasion and other forms of mechanical corrosion/wear, UR also protects assembly’s tin-surfaces, mitigating tin whiskers while limiting development of new short-circuits.  UR’s exceptional mechanical wear results from good moisture/humidity/chemical resistance and reliable dielectric properties.  Distressed by higher levels of heat/vibration, urethane films are prone to cracking/performance-failure over 125°C.

Requiring no curing, parylene coatings offer precise/uniform/flexible coverage regardless of assembly topography, with low dielectric constant.  Successful in the nanometer range, XY-coatings resist chemicals/corrosives/moisture/solvents, with minimal thermal expansion, maintaining PCB function/performance through most operational conditions.  Unlike liquid coatings under severe temperatures, XY doesn’t become brittle/decompose at upper-range temperatures, remaining adherent/intact, sustaining dielectric/insulation properties.  Ultra-thin, XY films  completely penetrate/preserve spaces at 0.01mm, making them superior for MEMS/nano-tech applications.  Providing tin whisker mitigation, parylene adheres to the widest range of substrate substances/surface geometries, factors that make XY the optimal coating choice despite higher cost/limited production throughput/difficult removal.

To learn more about how parylene compares to liquid conformal coatings, download our whitepaper:

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