Conformal coating is a protective nonconductive dielectric layer that is applied onto the printed circuit board assembly to protect the electronic assembly from damage due to contamination, salt spray, moisture, fungus, dust and corrosion caused by harsh or extreme environments.
Conformal coatings are usually used in products that are made for use in outdoor environments where heat and moisture are prevalent. Coating also prevents damage from rough handling, installation, and reduction of mechanical and thermal stresses. It also prolongs the life of the product during its operation. At the same time, it helps to increase the dielectric strength between conductors enabling the design of the PCB to be more compact and small. It also acts to protect circuitry and components from abrasion and solvents.
After the coating process is complete the coating is clearly visible as a clear and shiny material. Some coatings are hard, while others have a slightly rubbery texture. Most coatings include a marker that appears greenish white when viewed under UV light. This marker enables easy inspection of the coating thoroughness checking during production.
In the past, coatings were only applied to military and life/medical products as the cost and the process was significantly higher per unit. In recent years the development of materials and new processes has enabled a larger variety of items to be coated, including a wide array of consumer electronics products. As prices continue to drop, conformal coating will become increasingly common for circuitry and electronic components, especially as these items continue to shrink in size and dimension.
Conformal coating started as a simple process performed on electronic substrates in need of extra protection from external elements, with little attention paid to quality factors beyond adequate component coverage. The increased capability of semiconductor assemblies to perform complex tasks in automotive applications, traffic control, signage, outdoor surveillance, and mission-critical elements has increased the demand for conformal coating. Because device failure could have dire consequences, the quality of coating material application is critical. New equipment and processes are in place to accommodate the conformal coating requirements of these emerging technologies. A smooth transition to automated precision conformal coating can be achieved through an analysis of the product being coated and the desired result, the coating material used, the process selected, and of the economic costs over a period of time.
Different conformal coating types, such as acrylic conformal coating, urethane conformal coating, and silicone conformal coatings are frequently used in conjunction with different conformal coating systems to apply the coatings in a quick, easy manner for larger scale production.
Parylene is a transparent polymer conformal coating that is deposited from a gas phase in a vacuum. These polymers are polycrystalline and linear in nature, possess superior barrier properties, have extreme chemical inertness, and because of the deposition process can be applied uniformly to virtually any surface and shape including:
- Glass
- Metal
- Paper
- Resin
- Plastics
- Ceramics
- Ferrite
- Silicon
Acrylic Resin (Type AR):
Acrylic conformal coatings are easily applied. They dry to the touch at room temperature in minutes, have desirable electrical and physical properties and are fungus resistant. They have long pot life and low or no exothermic during cure, which prevents damage to heat sensitive components. Also, they do not shrink. The main disadvantage is solvent sensitivity, but this also makes them easier to repair.
Epoxy (Type ER):
Epoxy systems are usually available as “two-component” compounds. These rugged conformal coatings provide good humidity resistance and high abrasion and chemical resistance. They are, however, virtually impossible to remove chemically for rework because any stripper that will attack the coating also dissolves epoxy-coated or epoxy-potted components and the epoxy-glass printed circuit board itself. The only effective way to repair a board or replace a component is to burn through the epoxy coating with a knife or soldering iron.
Polyurethane (Type UR):
Polyurethane conformal coatings are available as single component, two components, UV curable, and water borne systems. As a group, all provide excellent humidity and chemical resistance plus outstanding dielectric properties for extended periods.
Silicone Type (SR):
Silicone conformal coatings are particularly useful for high temperature service, up to about 200° C. They provide high humidity and corrosion resistance along with good thermal endurance, making them desirable for PWA’s that contain high heat dissipating components such as power resistors. Silicone coatings are susceptible to abrasion (low cohesive strength) and have high coefficients of thermal expansion.
Source: Conformal Coating