Best Conformal Coating for Moisture and Chemical Protection

Conformal coatings safeguard printed circuit boards (PCBs) from performance malfunction caused by contact with elements within their operational environment, including moisture presence within the PCB, altering its thermo-mechanical properties and chemical incursion which can quickly corrode PCB components. In addition to mitigating these specific causes of PCB malfunction, conformal coatings also provide an overlying measure of basic protection, ensuring the PCB works as designed.

Each coating material possesses unique chemical/performance properties determining its value for moisture/chemical/basic assembly protection.

Moisture Protection

Disrupting a PCB’s thermo-mechanical properties and excess moisture can be caused by:

• Corrosion,
• Decreased glass transition temperature (Tg)
• Differential swelling/hygroscopic stress
• Diminished interfacial adhesion strength
• Metal migration/electrical-shorting

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

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 and de-wetted areas is imperative. These become sites of corrosion within the film and 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.

Epoxy’s (ER) hard conformal coating provides useful hydrophilic polar protection, but water migration remains possible, stimulating corrosion/lessened performance. Excessive moisture increases the 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.

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

Parylene is applied through a chemical vapor deposition (CVD) and provides the thinnest effective coating available and excellent substrate coverage. The exceptional uniformity of pinhole-free Parylene 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. ER is very durable and 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.

Properties	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 and 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. Its 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/nanotechnology.

Very resistant to abrasion and other forms of mechanical corrosion/wear, UR also protects the assembly’s tin surfaces, mitigating tin whiskers while limiting the 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, Parylene coatings resist chemicals/corrosives/moisture/solvents with minimal thermal expansion, and the coatings maintain PCB function/performance through most operational conditions. Unlike liquid coatings under severe temperatures, Paryelne does not become brittle/decompose at upper-range temperatures, remaining adherent/intact and sustaining dielectric/insulation properties. Ultra-thin, Parylene films completely penetrate/preserve spaces at 0.01 mm, making them superior for MEMS/nanotech applications. Providing tin whisker mitigation, Parylene adheres to the widest range of substrate substances/surface geometries, making it the optimal coating choice despite higher cost/limited production throughput/difficult removal.