Selecting a suitable conformal material for coating printed circuit boards (PCBs), related electronics, or other devices requires recognition of each coating type’s unique combination of benefits and drawbacks. These must be considered in relation to a given application.
Four liquid coating materials -- acrylic, epoxy, silicone and urethane – predominate. Liquid coatings are applied to substrate surfaces by the following methods:
· brushing the wet coating material onto the substrate,
· manually immersing (dipping) the PCB in a liquid bath of the coating substance,
· automated, programmed-coating application via robot, and
· manual spray application, using an aerosol can or spray booth.
Wet brush, dip or spray methods are relatively simple and inexpensive to enact. Effectively-applied liquid coating thoroughly covers all assembly surfaces, leaving no surface defects after flashing and curing. Common defects include:
· air bubbles,
· cloudy/hazy surfaces,
· irregular levelling (orange peel), or
· voids/breaches/gaps in the coating where the assembly’s surface topography is not completely flat.
Brushing is the application method of choice for epoxy prototypes and assemblies requiring a lot of masking. A buffer is recommended for use with more delicate components. Spray coating is acceptable for epoxy films; dip coating is not recommended.
A single- or two-part compound, epoxy resin (Type ER) conformal coating offers extreme hardness -- providing exceptional coating strength, dependably resisting abrasion, chemical incursion, humidity and vibration, with exceptionally rigid coating protection. Single-part compounds are cured thermally or by UV exposure. Two-part compounds begin to cure as soon as they are mixed together. Both systems provide the same benefits. They possess excellent temperature and chemical resistance. In addition to rugged toughness and durability, ER:
- is easily applied,
- features relatively high glass transition (Tg) temperature properties (50 - 90° C), while
- demonstrating good dielectric constancy (>3.0) and insulation resistance,
- protecting components from short-circuiting, with
- exceptional moisture barrier absorption-retention attributes (0.5 – 1.0%),
- moisture penetration levels 0f 27 – 30 gm/m2/day/25µ,
- remaining useful at temperatures of 150° C [302° F].
In addition, epoxy’s material costs are low in comparison to silicone, urethane and CVD-applied parylene; of major conformal coating types, only acrylic’s material costs are lower.
Two-part epoxy coatings provide long-lasting service on metal substrates; they consume less energy than heat-cured powder coatings, like parylene. These systems provide a tough, protective coating with excellent hardness. Some epoxy coatings are formulated as an emulsion in water, and can be cleaned-up without solvents. Due to reliable heat-resistance, epoxy coatings are often used in aerospace, automotive and industrial applications.
Other uses of epoxy conformal coatings include:
- adhesion-improving primers for metal surfaces subjected to corrosion, and
- metal cans/containers’ internal coating, to prevent rusting, especially for acidic foods like tomatoes.
While epoxy conformal films can provide reliable hydrophilic polar protection, the possibility of water migration beneath coatings during operation is a potential problem. Despite the unquestionable durability/hardness of epoxy’s outer layers, swelling within a coated region affected by water migration may generate excess water permeation, which can
- stimulate peeling of the conformal film,
- removing the assembly’s protective covering,
- subjecting it to infiltration by external elements,
- corrosion and performance degradation.
Using epoxy as a coating for components whose operating conditions are characterized by the presence of water, salt-spray and mist is not recommended.
While epoxy provides generally reliable exterior film protection for PCBs, its long-lasting surface durability also makes it very difficult to rework and repair; repair costs are high compared to most conformal coatings. Epoxy can shrink during polymerization. These two factors make exceptionally careful film application a necessity. Temperature extremes diminish its stress-resistance, further limiting epoxy’s use for electronics.
Except in the case of hermetically-sealed hybrids, highly chemical resistant epoxy conformal coating is difficult to remove from substrate surfaces with solvents, if component repair or film rework is needed. Difficult coating removal puts the coated product at risk of being damaged. The solvent can’t discriminate between the epoxy coating, the epoxy-glass PCB, and any epoxy-coated components. However, spot chemical-stripping can work, if care and time are taken to ensure safe removal, using a cotton-tipped swab of solvent with a methylene chloride base and acid activator. Micro-abrasion also works for epoxy removal.
Epoxy conformal film is most effective when total coating-layering is between 0.00118 - 0.00512 inches. If the coating is too thick, problems could include shrinkage, cracking, bubbling or trapped solvent. Other drawbacks include:
- Thermal extremes during operation significantly lower epoxy’s stress-resistance.
- Some epoxies lack UV trace, causing coating deterioration if over-exposed to UV-light.
- Mostly ineffective as protection against the development/impact of tin whiskers.
ER systems deliver extremely durable coating with good resistance to damage from chemicals, high-level abrasion, humidity and vibration. However, ER can shrink during polymerization, and temperature extremes diminish its stress-resistance. The same rugged, long-lasting films that distinguish epoxy performance makes them difficult to rework and repair, so application processes need to be enacted carefully.
To learn more about epoxy conformal coatings and how they compare to other conformal coatings, download our whitepaper: