Electronics manufacturing doesn’t just entail design and production. One critical part of electronics and PCB manufacturing is ensuring the safety of the board and electronic components. The evolution of technology has resulted in the exposure of electronics to several environmental conditions. Also, electronic components are vulnerable to mechanical and chemical degradation throughout their lifetime.
Commercial users and consumers raise their expectations as regards the durability of electronics. Also, they expect these electronics to be reliable. Therefore, manufacturers face pressure on how to deliver such electronics products. Hence, manufacturers developed means to shield PCBAs from any damage. Conformal coating is one of these protective measures applied by PCB manufacturers.
Any hazardous environmental condition can harm the functionality and integrity of components. Therefore, conformal coating has been a long lasting solution to damages caused by environmental hazards. Understanding what conformal coating is all about is crucial to its application in the PCB industry. Therefore, this article seeks to explore more knowledge on conformal coating.
What is Conformal Coating?
Conformal coating protects a printed circuit board and its components from any chemical damage. Also, this coating comprises a polymeric thin film that protects a circuit board from environmental contaminants. This coating is usually applied over the surface of a PCB. There are several environmental factors that affect the functionality of a PCB.
Also, conformal coating offers protection against environmental conditions and contaminants like moisture and chemicals. Furthermore, this coating prevents short circuits and conductors corrosion. It offers insulation that minimizes mechanical and thermal stress on electronic components. Conformal coatings are crucial in PCB manufacturing and are widely used across industries.
The primary objective of using conformal coatings is to prevent any exposure to corrosion. However, conformal coating offers more benefits like protection against electrostatic discharge. Conformal coating plays a crucial role in electronics and PCB manufacturing. Also, conformal coating is always in a liquid form before applying it to a PCB surface.
Conformal coating comprises several materials like epoxy, combinations of resins, and acrylics. Also, this coating offers a protective dielectric layer. Conformal coating is available in solder joints, uncovered traces, and electronic components. Due to its benefits, it is vastly used in several applications. Manufacturers apply conformal coating on top of electronic parts. Once applied, this coating creates a barrier between electronic components and dangerous materials.
Furthermore, conformal coating features a breathable protective layer. This layer enables any dampness trapped in the PCBA to dissipate. There are several types of conformal coatings.
Types of Conformal Coating
Conformal coating is available in different types. However, these coatings still carry out their major objective. It is important to understand the different types of conformal coating. Also, each of these coatings has its drawbacks and benefits.
Urethane coating
This type of coating offers high solvent resistance. Also, it is a type of high-strength hard coating. Urethane coatings feature excellent chemical resistance and increased dielectric resistance. Furthermore, polyurethane coating offers low moisture permeability. It is also abrasion resistant.
However, this coating type has its own drawbacks. Urethane resins don’t perform excellently in high-temperature environments.
Silicone conformal coating
Silicone coatings are soft and very transparent. Also, they provide great moisture and corrosion resistance. Silicone coatings have similar moisture resistance to urethane. They have great thermal shock resistance. Also, silicone conformal coating performs well at extreme temperatures. They feature light transmission characteristics. However, silicone conformal coatings don’t have high dielectric resistance.
Acrylic coating
This coating is solvent-based. Also, acrylic resin coating features anti-fungal properties. This type of conformal coating is transparent, durable, and hard. Acrylic coatings can be repairable since it is soluble in several kinds of organic solvents. Also, acrylic coatings feature fast drying properties. However, it doesn’t offer enough stress relief for circuit boards.
Epoxy conformal coating
This type of conformal coating is hard. Epoxy coatings resist the negative impacts of moisture and solvents. They are a two-part thermoset combination. Epoxy resin coatings are widely used due to their resistance to abrasion, chemical, and moisture. Also, it is difficult to remove this coating without harming the circuit board. This is because the thermoset mixture shrinks while curing.
Paraxylene coating
Paraxylene comprises linear and crystalline polymer coating. These polymers are conformal coating materials. This type of conformal coating is very transparent. Also, it can cover very narrow areas.
Application Methods for Conformal Coating
There are several ways of applying conformal coating. Some of these application methods include:
Atomised spray
You can apply conformal coating with the use of atomized handheld spray gun. This conformal coating application method is similar to spraying with aerosol. However, it can be more efficient and quicker. Ventilation is a crucial consideration in this method. Also, experts advise using a professional spray booth. Improper ventilation or wrong combination of material can result in problems.
Machine coating
This is a reliable method of applying conformal coating. Coating by machine is advisable if you have more cost to spare. Application of coating by machine will prevent some issues. Also, this conformal coating application method guarantees more accuracy.
Brush coating
Brush coating isn’t a reliable method of applying coating. There is a high degree of inaccuracy associated with this method even with a skilled technician.
Selective coating
Selective coating is the most efficient method of applying coating. However, this type of coating method is most expensive. Also, it allows technicians to choose the areas of the circuit board that needs coating. The technician will choose the required coating thickness. After this, the machine will perform its function.
Aerosol coating
Aerosol is a cost effective method of applying coating for small scale production. Also, this application method requires a well ventilated area. It is advisable to use specialized spray booth where needed.
Dip coating
In this method, the engineer dips the PCB into the coating. Dip coating is a slow process. Also, the solvent evaporates as the engineer withdraws the PCB. Dip coating can be a stressful process. This is because you will need to mask contacts in other areas you don’t want to coat. However, dip coating will ensure the PCB is entirely coated.
Importance of Conformal Coating in PCBs
Conformal coatings are crucial for printed circuit boards. This is because they protect PCBs from contaminants. For example, excess moisture can cause decomposition and result in conductor corrosion. Also, contaminants can result in unrecoverable shortcuts and electrical failures.
Conformal coating helps to ensure electronic products operate at peak efficiency. Also, this coating offers more benefits like:
Reduced maintenance and replacement costs
Conformal coating offers protection to PCBs. Therefore, it minimizes the damage sustained by circuit boards. Hence, this results in less maintenance or replacement of circuit boards. Also, by reducing the chance of damage, more time and money can be saved. This is because there won’t any need for much maintenance.
Protection from dangers
There are several damages circuit boards will suffer when exposed to some environmental conditions. The main objective of conformal coating is to protect circuit boards from such damages. Therefore, conformal coating protects boards from environmental hazards like moisture and heat. Also, conformal coatings shield PCBS from residues, dust, and other contaminants.
Improved inspection
Conformal coating also includes UV coating and tracers. These UV tracers help to evaluate the coating by using ultraviolet (UV) light source. The light helps to check any cracks, flaking or foreign matter in the coating.
Extreme thinness
PCBs and devices have weight and thickness restrictions. Conformal coating offer great benefit. This is because it is thin enough to fit on PCBs and devices with the strictest restrictions.
Curing Methods of Conformal Coating
The curing method determines the type of application method to use. Some curing mechanisms are infallible. However, some are more complex and have more application errors when applied in an uncontrolled process. Also, cured coating offers higher chemical resistance.
Heat curing
Heat curing is ideal for use with one or more component systems. This method is a secondary mechanism for evaporative curing, UV curing, and moisture curing. Also, the use of heat will make the system to polymerize. This can be beneficial when a curing mechanism isn’t enough to achieve the curing properties required. However, one must consider the thermal sensitivity of PCBs and components when curing at high heat.
Evaporative curing
Here, the liquid carrier evaporates and leave the coating resin. Engineers need to dip PCBs at least two times to create enough coating on the edges of their components. Humidity impacts application parameters whether the liquid carrier is water or solvent-based. Solvents can be flammable. Therefore, enough ventilation is necessary. The use of water as a carrier can get rid of the flammability concern. However, these coatings take longer time to cure.
UV curing
Ultraviolet light provides fast throughputs when used to cure conformal coatings. UV cure has no carrier solvents. Also, UV cure happens during production. Therefore, you will need a secondary curing method in shadowy areas. However, UV coatings are difficult to rework and repair. Also, they require UV radiation protection for workers.
Moisture curing
This curing method is commonly found in some urethane systems. Moisture curing is usually coupled with evaporative curing. Moisture will react with resin as carrier solvents dissipates.
What is Conformal Coating Removal?
Conformal coating removal is crucial in circuit boards. This helps to replace damaged components. There are materials and methods needed for coating removal. The size of the area and the coating resins determine these methods and materials. According to the IPC, the basic coating removal methods are:
Peeling
You can remove some conformal coatings from the PCB by peeling it. This is usually ideal for some silicone conformal coatings. Also, it is ideal for other flexible conformal coatings.
Grinding or scraping
This conformal coating removal method is very effective. It removes conformal coating by abrading the PCB. Also, this method is more ideal for like polyurethane and parylene coatings.
Solvent removal
Acrylics are the easiest coating removal. Most coatings are prone to solvent removal. Therefore, you must determine if solvent won’t damage the components of the PCB.
Microblasting
This coating removal uses a mix of soft abrasive and compressed air to remove coating. The mixture will abrade the coating. Also, this process can help to get rid of small areas of conformal coating. It is widely used in parylene and epoxy coatings removal.
Conformal Coating Process
There are several steps involved in production of conformal coating.
Cleaning
This is the first step of conformal coating process. Cleaning helps to remove different residues that result from soldering, and operator handling. Also, it removes residues caused by environmental contamination. Plasma cleaning, solvent cleaning, and aqueous chemistries are cleaning techniques used in this step.
Priming
Priming involves the application of a preparatory coating known as primer on a PCB. This step helps to ensure formation of a binding layer to absorb the coating. The type of resin determines the requirement for priming. Acrylic resin coating don’t need coating. However, silicone conformal coating uses primers to improve the process.
Masking
Masking prevents the application of conformal coating in components and designated board areas. Also, certain areas on the PCB must be uncoated. Therefore, selecting the right materials is crucial in avoiding masking failure.
Application
This step involves the use of different application methods to apply conformal coatings. These methods include dipping, brushing, and more.
De-masking and finishing
After applying the coating, masking removal is necessary. Finishing is the next step after de-masking. Also, finishing helps to prevent masking leakages and coating defects. In addition, it guarantees the quality of coating and verifies standards.
Drying and curing
Drying ensures the coating of the circuit board and makes it ready for operators to handle. Also, this process can take some minutes to several days. Curing helps to ensure the coating gets its necessary properties and shields the PCB during operation.
Inspection
Inspection is the most crucial step in conformal coating. This step includes UV lamp visibility, AOI, and manual inspection.
Certifications and Regulations for Conformal Coating
There are regulations and certifications that apply to the use of conformal coatings. Also, certifications distinguish varnishes and shellacs from coating specially designed for PCB protection. There are several specifications for conformal coatings. However, the UL746E and IPC-CC-830B are the two major certifications for conformal coating.
UL746E
The Underwriters Laboratories (UL) is a safety certification organization that regulates the safety of consumer electronics. The UL746E test checks the fire and electrical safety in coated electronics. If a conformal coating passes this test, the PCB has a UL registration under a specific number. Also, any PCB with approved coating must be annually retested to maintain registration.
IPC-CC-83OB
IPC-CC-83OB was once a military standard before it transitioned into a civilian version. This test majorly focuses on the appearance, stability, and flammability of a coating. Materials that meet MIL-I-46058C standard will automatically meet the IPC-CC-830B requirements. Also, the IPC-CC-830B is continually being updated.
IEC 61086
This standard works based on supplier self-certification. The International Electrotechnical Commission (IEC) is a body that regulates the use of conformal coatings in circuit boards.
UL94V0
This standard checks the self-extinguishing and flammability nature of conformal coating on FR4. The highest classification you can achieve is the V-0. However, they are other categories like V-2 and V-1.
Conformal coatings must meet certain regulatory standards for environmental safety. The Occupational Safety and Health Administration (OSHA) is a body in the United States that set regulations for conformal coatings.
Furthermore, the Environmental Protection Agency (EPA) guidelines are applicable. This safety certification body regulates the use of environmentally hazardous chemicals in PCBs. Also, the standards mentioned above regulate the productions of coated electronics.
What are the Risks Involved when Conformal Coating isn’t Used in PCB?
Conformal coatings reduce several risks when used in circuit boards. This is one of the reasons these coatings are widely used in PCB manufacturing. Also, you expose PCBs to various dangers when you don’t use conformal coating. These dangers would cost your time, reputation, and money. Below are some risks involved in the failure to use conformal coating.
More replacement expenses
Replacement expenses can cost a fortune. Failure to coat a circuit board will expose your board to many dangers. Also, these dangers will end up damaging your board. In the long run, you will spend more money on maintenance or replacement.
High repair costs
When a circuit board starts malfunctioning, you will start planning on how to get new materials. Purchasing new materials to fix this board will cost you a lot. if you fail to coat your PCB, you will repair them often. Therefore, you will incur more costs.
Loss of reputation
A PCB manufacturing company can lose its reputation if it fails to apply conformal coating. Also, customers will avoid purchasing your products once they notice this.
Important Considerations when Selecting the Right Conformal Coating for PCBs
Since conformal coating is available in different types, you need to choose the right coating for your PCB.
Operating Environment
Conformal coating helps to protect a printed circuit board from any form of corrosion. Therefore, it is crucial to consider the ability of the coating to protect the PCB from its environment. Hence, you will need to consider the temperature range and the humidity of the PCB environment.
Also, the coating has to be ideal for use in some operating conditions. Determining the level of daylight exposure is important. Furthermore, you will need to know how quick the operating temperature changes.
Electrical requirements
Conformal coating usually creates an insulating layer. Therefore, the Surface Insulation Resistance is the commonest electrical parameter tested. Also, it is important to take this measurement before and after coating. Therefore, the coating must offer the required level of insulation. In addition, the coating should have high dielectric strength.
Production volume
The production volume will determine the type of coatings to opt for. Also, the production volume will impact other factors like the cure times and application method. For instance, automated application is more suitable for mass production. However, hand spray coating will be suitable for small production volumes.
The PCB design should consider the components that don’t need coating. You can apply selective coating method to avoid contact with such areas. Also, you will need to consider the proximity of the components. Insufficient gap between electronic components can make the coating bridge the gap.
Repair and rework
If the PCBA needs repair, you need to consider the ease of coating removal. There are products that help to remove conformal coatings easily.
Most Common Defects in Conformal Coating
There are some common defects associated with conformal coating. These defects in most conformal coatings include:
Air bubbles
As you mix the spray system, you can introduce air in the coating material. Also, brushing can trap bring in air bubbles into the coating.
Dewetting
The contamination on the substrate can cause dewetting. Contamination like fingerprint oils and flux residues are not compatible with the coating. Also, you can solve this problem by thoroughly cleaning the substrate.
Cracking/crazing
Excessive film thickness can cause long cracks. Also, insufficient time between overcoats can result in crazing. Curing with heat acceleration can result in crazing and cracking. A lower temperature for a longer time can solve this.
Fisheyes
Fisheyes are small circular area in the center. Also, it is usually seen after or during spray application. Water or oil in the sprayer air system can result in fisheyes. A well-maintained filtration system that removes any moisture from getting to the sprayer will prevent this defect.
Orange peel
This defect occurs when the air pressure when using a spray system is low. This results in the production of a texture that looks like orange peel. Application of many heavy wet coats can cause orange peel. Also, the incorrect application of thinner can make coating evaporate quickly. Therefore, this will prevent the thinner from taking enough time to level out.
Conclusion
Conformal coating is an integral aspect of PCB and electronic manufacturing. It is one of the best ways to fabricate durable and reliable PCBs. Also, this coating features excellent mechanical and electrical properties. Conformal coatings are very thin. Therefore, the cured coating thickness is usually between 1 to 5 mils. Ensure you take some factors into consideration when choosing a conformal coating.
