The Role of Conformal Coating in Printed Circuit Board Assembly

Coating in Printed Circuit Board Assembly

The durability and reliability of printed circuit board components are vital to the success of electronic products. However, a multitude of environmental threats can compromise the performance and lifespan of these delicate devices. Conformal coating plays a critical role in protecting PCBs from dust, humidity, chemicals and extreme temperatures. It also protects against corrosive elements, such as salt water and corrosive gases.

Different types of conformal coating exist on the market, each with unique properties and characteristics. The most common are acrylic, silicone, polyurethane and parylene coatings. Each has its own benefits and uses, but the best one for a particular application depends on the type of environment in which the printed circuit board assembly will operate.

Electrical insulation is an important attribute that conformal coating must provide, and this is typically tested by measuring the Surface Insulation Resistance (SIR) of the coated area. This measurement is taken before and after the conformal coating process to assess its performance.

The Role of Conformal Coating in Printed Circuit Board Assembly

Achieving a high SIR value helps to ensure that the conformal coating will continue to function in harsh environments, and it is especially important for mission-critical electronics. SIR testing is performed to a variety of ISO standards, and it is recommended that manufacturers test their materials to these guidelines.

The type of application method used will also impact the quality of the coating. Manual spraying methods, such as aerosol spraying and batch spraying with a spray gun, are more cost effective for low volume production but can be time-consuming since areas not requiring coating must be masked. Selective spraying allows for more accurate coverage but requires specialized equipment and training.

Once the coating is applied, the circuit boards must be thoroughly inspected. This can be done manually using a UV lamp or by automated optical inspection (AOI) systems. The latter have the added benefit of capturing data on each PCB as it is processed, providing a more streamlined and reliable inspection process.

All conformal coatings must meet safety and regulatory standards, and it is essential that the resulting material is non-hazardous to both workers and the environment. This is particularly true in the United States, where Occupational Safety and Health Administration (OSHA) and Environmental Protection Agency (EPA) guidelines must be adhered to.

Depending on the type of conformal coating used, it may be necessary to perform additional tests to verify its suitability for a specific environment. For example, a salt mist test is often required for applications that will be exposed to marine conditions. This test exposes the circuit boards to a mix of salt and water, which is very difficult for most conformal coatings to resist, and it is necessary to pass this test before a circuit board can be shipped out to a customer.

In order to prevent pinholes from forming and making contact with the circuit boards, the first step in applying conformal coating is to remove all contaminants from the surface of the board. This is done with a repeatable industrial process, such as vapor degreasing or semi-aqueous washing. These steps are often the most costly, but they are vital to ensuring that the conformal coating will work as expected and provide sufficient protection.