PCB Conformal Coating

As modern household appliance industry pays more and more attention to the product function reliability, conformal coating the concept is being mentioned by manufacturers more and more. There are many different facets to conformal coating, from equipment and applications to the different types of coatings. Conformal coating is a protective non-conductive 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 environments where heat and moisture are prevalent. Coating also prevents damage from rough handling, installation, and reduction of mechanical and thermal stresses.

Applying conformal coating to form an insulating and moisture-proof layer on the surface of circuit board, fix dust and particles to avoid short circuit, package components to reduce contact with the environment and prevent corrosion, protect metal contacts in electronic devices from environmental damage, which makes a qualitative leap in the environmental reliability of the product.

From chemical composition, conformal coating binder can be divided into acrylate and silicone. Acrylic ester is a binder with high hardness, high adhesion strength and excellent solvent resistance, but it’s high temperature resistance is limited. Silicone as a matrix binder, it has excellent high temperature resistance, good flexibility, and it’s easy to repair, but it’s adhesive strength is limited.

From the curing mode, there are solvent curing, heat curing and UV curing and other processes can be selected.

Development

Conformal Coating 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.

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.

Thickness

Conformal coating thickness can be critical to the proper function of a PWA. If a coating is too thin, proper coverage is impossible; if a coating is too thick, it may create excessive stresses on solder joints and components (particularly glass-bodied components). Controlling coating thickness is of special importance with rigid coating materials (e.g., epoxies and some of the urethanes) because the residual stresses associated with an excessively thick application of these materials are much greater than with flexible coating materials (such as silicones and some urethanes).

Conformal coating thickness requirements are in accordance with IPC J-STD-001 for the following coating types:

Additionally conformal coating thickness is measured on a flat, unencumbered, cured surface of the printed circuit assembly or a coupon that had been process with the assembly. Coupons may be of the same type of material as the printed board or may be of a nonporous material such as metal or glass.

Conformal coating thickness/coverage shall have the following characteristics to ensure that coating thickness specifications are upheld:

• Be completely cured and homogeneous.

• Cover only those areas specified on the assembly drawing/documentation.

• Be free of blisters, breaks that could affect the operations of the assembly or sealing properties for the conformal coating.

• Be free of cracks, crazing, voids, bubbles, mealing, peeling, wrinkles or foreign material which expose component conductors, printed circuit conductors, or other conductors and/or violates design electrical clearance.

• Performed per IPC-7711 and IPC-7721 Specifications.

Hope this is a good description of the different types of Conformal Coating.