What are Conformal Coatings?
Conformal coatings are thin protective layers or films that are applied to the surface of printed circuit boards (PCBs) and other electronic assemblies to protect them from a variety of environmental hazards such as moisture, chemical contamination, dust, etc. These coatings provide a physically continuous and mechanically robust covering that fills the crevices and flows into gaps on the circuit board surface.
Types
There are various types of coatings used in the electronics industry based on their composition and properties. Some of the commonly used types are:
Acrylic Conformal Coatings
They are thermoplastic coatings made from acrylic or methacrylate polymers. They offer excellent moisture protection and resistance to chemical permeation. Acrylic conformal coatings have good moisture barrier and dielectric properties along with good abrasion and corrosion resistance. Some drawbacks include low thermal resistance and chemical susceptibility under prolonged exposure.
Silicone Conformal Coatings
They are based on silicone polymers and elastomers. They offer very high flexibility and thermal resistance along with excellent moisture barrier properties. Silicone Conformal Coatings have good dielectric strength and remain stable at temperatures ranging from -55°C to 200°C. However, silicone coatings may degrade or yellow with exposure to sunlight and are susceptible to water spotting in high humidity environments.
Urethane Conformal Coatings
They have high abrasion and impact resistance along with good adhesion and flexibility properties. They can withstand temperatures up to 125°C. Urethane coatings provide effective moisture and chemical barrier even under harsh conditions. The main disadvantage is their lower dielectric strength compared to silicone or acrylic coatings.
UV Cured Conformal Coatings
UV coatings are 100% solids liquid coatings that are cured using ultraviolet light. They offer advantages like no volatile organic compound (VOC) emissions, rapid cure times and good physical properties. UV cured coatings are more environment-friendly than solvent-based coatings. However, they have lower thermal resistance and may not be suitable for applications requiring temperatures exceeding 125°C.
Epoxy Conformal Coatings
They are thermosetting polymers that form very hard and durable surfaces after curing. They offer excellent chemical and moisture resistance along with high dielectric and abrasion resistance properties. Epoxy coatings can withstand temperatures up to 150°C. However, they have lower flexibility compared to other coating types.
Processes
There are various application methods used for them depending on the assembly design and coating properties:
Spraying
Spray coating either manually or using automated equipment is a commonly used technique. It provides for uniform coverage of multilayer circuit assemblies. Both air-atomized and air-assisted airless spraying systems are used.
Dipping
Dipping involves fully submerging the whole PCB assembly in a liquid material inside an enclosure. It allows for simple, reproducible application but may require subsequent touch-up depending on the geometry.
Brush/Swab Coating
Brush or swab application is suitable for small assemblies or complex geometries. It allows for controlled deposition but may not provide uniform thickness over large areas.
Selective Coating
For functional separation of circuits, selective coating techniques like screen-printing or selective spraying using masking are adopted to coat only specific areas.
Benefits
They provide many advantages for circuit protection and reliability:
1. Moisture and Environmental Protection: They act as an effective moisture barrier, preventing corrosion and shorts caused by moisture penetration. This improves the operational lifespan of electronics even under humid conditions.
2. Chemical Protection: The protective coatings prevent chemical contamination from atmospheres containing salts, solvents, corrosion-causing gases, etc. ensuring long-term performance stability.
3. Abrasion Resistance: The robust coating layer prevents mechanical damage to components and printed wiring from handling, cleaning processes, abrasive dust particles, etc.
4. Dielectric Strength: They help maintain the dielectric integrity of the circuit boards providing insulation between components. This ultimately improves reliability.
5. Flexibility: Flexible coatings like silicones accommodate thermal expansion and component flexing without cracking or delamination from the PCB surface.
6. Lower Cost: They eliminate the need for more expensive encapsulation, thereby reducing overall cost and ensuring functionality.
7. Process Dependability: Consistent coating application and the ability to touch-up coat defects makes conformal coatings a very dependable manufacturing and repair process.
they provide multi-functional protection at low cost, making them an indispensable part of electronics manufacturing assembly lines globally. The ability to coat complex circuitry conformally makes them vital for harsh environment electronics applications.
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