Conformal Coating on PCBs

A Conformal Coating protects the circuit board assembly in harsh environments and from mechanical influences like vibration and shock. Such protection greatly increases the life of printed circuit board assemblies.

What is Conformal Coating for PCBs?

We apply conformal coatings to PCB assemblies for protecting the components and the surface of the board from contaminants like conductive particles, salt spray, moisture ingress, sand, and dust. The coating provides protection from mechanical influences and environmental effects, thereby substantially increasing the durability of the printed circuit board and components thereon.

The term conformal is a derivative of the Latin word 'conformis'. It defines the ability of a coating to follow the shape of the surface on which the operator applies it.

Although conformal coatings are only a few microns thick, they also provide resistance to various external factors like impact, friction, temperature, and degradation from UV radiation and ozone. This improves the performance of the PCB assembly and substantially increases the dielectric strength between adjacent traces on the board, thereby allowing higher component density than that possible in regular boards.

Types of Conformal Coatings and their Application

Typical conformal coatings meant for printed circuit board assemblies are either silicone or acrylic based. However, other types based on urethane, epoxy, parylene or parksilene and UV curable are also in common use.

Conformal coatings are applied on boards by watering, spraying, or dipping. Manufacturers also use robotic dispensers when handling a large number of PCB assemblies. Traditionally, solvent-based coatings methods were used that took hours to cure. The latest coatings are solvent-free, meant to cure quickly in minutes with the application of gentle heat.

Conformal coatings are useful for application on common types of PCBs made of fiberglass, but they are equally applicable on boards made of phenol or Teflon with multiple layers and components on both sides.

How Conformal Coatings Protect the Board?

The protective conformal coating protects the surface of the board and components of the assembly from external influences during operation. This includes environmental influences like pollution, moisture, high temperature, chemical and mechanical effects. Apart from this, conformal coatings also help to increase the mechanical strength of solder joints, and reduce electromigration.

A properly applied conformal coating has optimum viscosity and strong adhesion, allowing it to cover the surface of the board and penetrate between the pins of the components. It finally conforms to the shape of the finished assembly, protecting it from all sides.

After application, it is necessary for the conformal coating to undergo a curing operation. This requires application of UV radiation, infrared radiation for heat, and good ventilation for convection. Some coatings are curable with UV irradiation alone, not requiring the application of heat.

To help visually inspect the integrity of the conformal coating, the material under the coating usually glows under blue light.

Standard for Quality Control of Conformal Coatings

Control of the application process of conformal coating is a crucial aspect of its success. It helps to follow an international standard like the IPC-A-610, the Acceptability of Electronic Assemblies. It is important to understand the meaning of the provisions in the standard, and explore the possibilities of new technical means, especially automated, for controlling the quality of the deposition. This includes factors that the applicant must take into account for ensuring reliable control of the process.

A study of the IPC-A-610 standard facilitates the understanding of the operator’s needs and requirements of the coating process. There are three sections in the standard:

• General Information on Conformal Coatings
• Coverage completeness
• Coating Thickness

According to the IPC-A-610 standard, the conformal coating must be transparent and uniform in consistency and color. It must coat the PCB assembly evenly. The application method decides the coverage.

Conformal Coating Technology

Characteristics of the conformal coating depend on the technology of the application, which can be by manual brushing or a robotic application with an aerosol or airless valve spraying. The technology of the application gives different levels of the finish, and this again varies depending on the actual process itself, the skill of the operator, and the conditions of the environment in the shop.

The term uniformity of the coating is to be considered with the completeness and thickness of the coating. Although the standard calls for a transparent coating, many customers prefer the coating to be colored.

Conformal coatings typically contain UV luminescent additives.

UV luminescent additives make it easier to control the coating quality.

Some defects are not visible in UV light, and require white light.

Some organic silicone coatings do not contain UV luminescence, and these are difficult to control.

The Conformal Coating Process

The IPC-A-610 sets targets for the quality of the conformal coating. These include:

• No areas with loss of adhesion.
• No bubbles or voids.
• No exfoliation, cracks, wrinkles, ripples, fish eye, or orange peel defects.
• No foreign inclusions.
• No loss of transparency or discoloration.
• Complete curing and homogeneous structure.

However, all coating technologies, types of PCBs, and materials cannot practically achieve all the above targets. Moreover, striving to achieve all the targets simultaneously may be expensive, both financially and investment-wise. The time and efforts spent on process control may make the process unviable.

The quality control criteria set by IPC for the specified targets states:

• The coating must be fully cured and have a uniform structure.
• The coating must be present only in those areas where it is necessary.
• The coating must have adhesion near the masked areas.

Thickness of Conformal Coating

According to the IPC-610-A standard, the allowable dry film thickness is in the range of 30-130 µm or 0.03-0.13 mm. Although this is a wide range and easy to achieve if the process is set up properly, it is equally easy to overshoot the specified limits.

Conclusion

Use of automated optical inspection systems to identify and control all defects when applying conformal coatings. We implement an iterative process for developing the optimal solution.

Author Bio:

RUSH PCB Inc has 20 years of experience in electronic contract manufacturing. Click here to learn more about RUSH PCB.