Technology’s evolution has resulted in electronics being exposed to a range of unique environments. As a result, sensitive components and connections can be exposed to environmental hazards such as extreme temperatures, moisture, corrosion and dust. Any of these environmental conditions harm the components’ integrity and functionality. For this reason, manufacturers employ protective measures to secure the integrity of printed circuit boards (PCBs). One protective measure is conformal coating.
What is conformal coating and what is it used for? In short, conformal coating is a thin, transparent film that can be applied to the surface of a circuit board. This film contours to the PCB’s shape, protecting components from environmental conditions without impacting functionality.
But is conformal coating necessary? While not all applications are best served by conformal coating, it can be an excellent choice for specific types of PCBs. Some of the benefits conformal coating offers include the following:
- Protection: Conformal coatings protect assemblies from environmental hazards such as chemicals, corrosion and dust.
- Lightweight: PCB conformal coatings do not significantly increase the board’s weight.
- Insulation: Conformal coating provides insulating properties, allowing a reduction in PCB conductor spacing.
These benefits are all provided without needing sophisticated or highly designed protective elements.
How Are Conformal Coatings Applied to Circuit Boards?
Conformal coatings may be applied to circuit boards using various methods, which differ in quality level, reliability and production level. The most notable application methods include the following:
- Brush method: In this technique, conformal coating materials are applied by hand using a brush. These coatings tend to be thick and less consistent than those applied by other methods. This process is simple and labor-intensive and best suited for low-volume production and rework and repair operations.
- Spray method: The spray method involves using an aerosol spray to apply conformal coating materials to PCB surfaces. Ideal for low-volume production, this technique provides an excellent surface finish consistency but is also time-consuming as it requires a thorough application to cover the entire board.
- Dipping method: The dipping method involves applying conformal coating to circuit boards by submerging them in the coating solution. Fast, accurate and thorough, this method is ideal for high-volume production. However, it is only useable on certain types of PCBs with designs that prevent leakage and can be coated on both sides of the board.
- Selective coating: The selective coating method uses robotic spray nozzles to apply coating materials to specified areas of a PCB assembly. This automated process is ideal for high-volume applications, offering a fast and accurate procedure that can be used on a wide range of boards. Like the dipping method, the board must be designed to be compatible with selective coating.
With each of these application methods, the conformal coating must be very thin to minimize heat entrapment and weight increase, among other concerns. Typically, conformal coatings are between 3 to 8 mil thick. How do you measure conformal coating? This is usually done with a micrometer, current probe or specialized gauge that uses physical measurements, electromagnetic fields or ultrasonic waves to take measurements of the coating after it is dry.
Types of Conformal Coating
Many types of conformal coating materials exist, including a range of specialty varieties. The most common of these conformal coatings are described below, along with their most prevalent applications:
- Urethane resin: Urethane resins (UR) or polyurethane resins are known for their excellent resistance to moisture, abrasion and chemical attacks. The downside of this material is that it is resistant to solvents, meaning it is difficult to remove and rework. Urethane resins are most commonly used in aerospace applications where components may be exposed to corrosive fuel vapors.
- Acrylic resin: Acrylic resins (AR) are acrylic polymers dissolved in a solvent. These substances require a simple drying process and are easy to rework. When complete, this type of coating provides good general protection against humidity and other environmental factors but poor protection from solvents and chemical vapors, making it most suitable for basic-level protection. The major upside of this material is the ease with which it can be removed and reworked, making it practical for repair operations.
- Epoxy resin: Epoxy resin (ER) coatings are compounds that create a hard layer with good humidity, abrasion and chemical resistance, with minimal permeability. Less flexible than other types of coatings, epoxy resins are difficult to remove and rework. Epoxy is the most common choice for PCBs that need to be coated completely and is often chosen for applications with low mechanical stress.
- Silicone resin: Silicone resin (SR) coatings provide excellent thermal, chemical, moisture and corrosion resistance while maintaining good flexibility. This type of coating is difficult to remove and offers poor abrasion resistance due to the surface’s rubbery texture. However, it makes up for this in offering good resilience against vibrational stress. Silicone resin conformal coatings are often chosen for electronics in outdoor environments that are exposed to broad temperature and moisture conditions.
- Parylene: Parylene (XY) coatings are applied through chemical vapor deposition. In this process, the parylene is heated to become a gas and put into a vacuum chamber to polymerize and turn into a thin film, which is placed over electronics. This film offers excellent dielectric strength and resistance to extreme temperatures, moisture and corrosive elements. However, it is also difficult to remove and rework, requiring abrasion techniques to remove. The specialized production process also makes replacing parylene coating more difficult than other methods. It’s often used in specialty applications.
When considering what types of conformal coating are best suited for your application, consider the application’s requirements, including the required functionality and environmental conditions in which the PCB will operate.
Certifications and Regulations
Another thing to consider with conformal coatings is the certifications and regulations that apply to them. Certifications distinguish general-purpose coatings from those specifically designed for PCB protection. The two major certifications in this area are IPC-CC-830B and UL746E, which are described below. Many coatings will have one or both of these certifications, which indicate the coating’s safety for use on PCBs:
- IPC-CC-830B: IPC-CC-830B started as the military standard MIL-I-46058C, then transitioned into a civilian version. It is common to see testing facilities refer to one or both of these standards interchangeably. It primarily focuses on the coating’s appearance, flexibility, flammability, stability and resistance to insulation, fungus, moisture and thermal shock.
- UL746E: UL746E tests are standardized by Underwriters Laboratories (UL), a reliable global safety certification body that regulates safety for consumer electronics. UL746E tests specifically look at the electrical and fire safety of coated electronics. Once a coating passes UL746E, it is registered with UL under a specific number and must be retested annually to maintain registration.
In addition to certifications, PCB conformal coatings must also meet regulatory standards for environmental and worker safety. Some of the most significant regulations that apply to PCB conformal coating in the United States include guidelines from the Occupational Safety and Health Administration (OSHA), which focuses on worker safety. Additionally, Environmental Protection Agency (EPA) guidelines also apply. This organization regulates environmentally hazardous chemicals.
Local agencies may have additional regulations for manufacturers to follow that may be more stringent than those enforced at the federal level.
Our PCB Manufacturing and Assembly Services
When choosing a conformal coating for your printed circuit board, it’s essential to keep the application’s requirements and relevant regulatory agencies in mind. If you’re looking for a manufacturing company that can help you choose and apply the right materials, EMSG, Inc. can help.
EMSG is an electronic manufacturing company in York, Pennsylvania, that offers a full range of PCB manufacturing and assembly services backed by 30 years of experience. By partnering with EMSG, you can enjoy the benefits of quality project assistance and top-of-the-line manufacturing practices, including conformal coating services.
Learn more about EMSG and our assembly, testing and manufacturing capabilities today by calling our offices at 717-764-0002 or contacting us through our website.