Conformal coatings consist of various polymeric materials used to protect the function and extend the life of electrical and mechanical circuitry, parts and related components. They safeguard parts and products from environmental contamination during use, insulating the substrate while doing so.
To fulfill this objective, they are applied in minute layers, typically in the range of a few millimeters, directly onto selected substrates. Most conformal coatings -- acrylic, epoxy, silicone, urethane -- are applied by dipping, brushing, or spraying, although select-coating or robotic dispensing are increasingly used. A major exception is parylene, which is applied through a vapor deposition process.
Operational security generated by conformal coating includes protection from:
- Contamination caused by exposure to harsh physical environments, extreme temperatures and humidity,
- Abrasion during performance and contact with acids, solvents or, in the case of medical equipment, bodily fluids,
- Conductor electro-migration, corrosion, dendritic growth, or short circuits to electronic assemblies and circuitry,
- Coatings also provide stress-relief and insulation to assure ongoing functionality.
This level of protection has been adapted to wide range of product applications, encompassing aerospace/military technology. consumer electronics of all kinds, harsh automotive mechanical applications, industrial uses, microelectromechanical systems (MEMS), and medical products that operate either within or attached to the human body. Indeed, without conformal coatings, many of the products we commonly take for granted would not function nearly as well, and would need to be replaced far more frequently.
Parylene: The Distinctive Conformal Coating
Parylene provides excellent substrate coverage, offering the thinnest effective coating application available in comparison to other conformal coatings. The vapor deposition process penetrates into the substrate surface, generating the highest levels of protection available for many products. Parylene surfaces are exceptionally resilient, withstanding extremes in temperature and physical stress. Their consistency remains very uniform, yielding excellent pinhole coverage that prevents leakage. Parylene's exceptional dielectric properties make them the covering of choice for a wide array of electrical components. However, reliance on the vacuum application technique for application makes their manufacturing cost higher than other conformal coatings. Also, parylene can be sensitive to contamination.
Other Conformal Coatings
Properties of the other conformal coatings include:
- Acrylic: Drying rapidly after application (via brush, dip, spray methods), acrylic coatings do not shrink during cure and persistently exhibit good fungus and humidity resistance in use. However, they often break-down more readily at higher temperatures than other polymers, and have limited abrasive and stress-relieving capabilities. Their cost advantage compared to other coatings has declined in recent years.
- Epoxy: Unlike acrylics, epoxy resins have good abrasive and chemical resistance. They are applied similarly to acrylics and demonstrate good humidity resistance. However, film shrinkage during polymerization is common, which somewhat counteracts the effectiveness of the extremely durable coatings they provide. In addition, thermal extremes significantly lower their stress resistance.
- Silicone: Able to withstand extreme differences in temperature, silicone has a useful operating range of -55°C - +200°C. Its dissipation factor is low, and it exhibits good PCB-adhesion, as well as high levels of resistance to heat, humidity, moisture and ultraviolet light. Very versatile, silicone conformal coating can be customized according to a product's precise requirements, with surfaces ranging from elastomeric, stress-relieving coverings to those far more abrasion-resistant and durable. Their toxicity is low and they are easily repairable.
- Urethane: These coatings have excellent dielectric properties, good chemical resistance, low moisture permeability, and good temperature flexibility at low temperatures. Although they are tough and display dependable resistance to solvents, their bond-strength is limited; urethane coatings covering larger areas have a tendency to flake and peel. This factor largely negates urethane's fine capacity for abrasion resistance; high-temperature resistance and repairability are also limited.
Each conformal coating has its own unique properties which dictate its particular range of product uses, and, for instance, the required coating-thickness necessary to assure reliable performance. These conditions vary according to product and purpose.
As an example, NASA Standard (NASA-STD ) 8739.1a for space flight and exploration systems requires a specific coating-thickness for each coating material. These are levels of coating that assure reliable, safe performance of circuits and components under often-extreme conditions. When assessed by these standards, thinner layers of parylene provide equal or superior protection, compared to other conformal coatings (measured in millimeters (inches)):
- Parylene -- 0.013 – 0.051 (0.0005 to 0.002).
- Silicone -- 0.051 – 0.203 (0.002 to 0.008).
- Acrylic, urethane, epoxy -- 0.025 – 0.127 (0.001 to 0.005).
Parylene displays similar coating-thickness and temperature advantages for most conformal coating projects. Each of the major types of conformal coatings offers particular advantages for a range of uses. All are less costly and easier to apply than parylene, but none displays parylene's versatility of uses -- from aerospace/military to medical, consumer goods to automotive, for MEMS and technologies waiting beyond. Of available conformal coatings, parylene withstands specialized and often harsh environments with optimal functionality to the most reliable degree.