Everything you need to know about PCB assembly conformal coating

A conformal coating is a type of thin polymeric film applied to a PCB (printed circuit board) by machine or by manual in order to have the electronic PCB boards and its components to be protected from the environment and corrosion. Normally, the film is applied at typical thickness of 25-250µm and ‘conforms’ to the shape of the PCB board and its assembled components, protecting and covering solder joints, the leads of electronic components, exposed traces and PADs, and other metallised areas from corrosion, so as to extending the working life of the PCB, and strength the electronic boards more reliability and safety.

Automatic PCB assembly conformal coating process

Why do PCB need Conformal Coating?

Normally, any newly manufactured printed circuit board with electronic components assembled on can generally perform working well, but this performance would quickly deteriorate due to external harmful factors in its operating environment. In order to defend the hazardous factors, the conformal coatings is commonly used in a wide range of environments to protect printed circuit boards from moisture, salt spray, chemicals, dusty, and serious temperature in order to positively prevent such issues as corrosion, quick aging, mould growth and electrical failures. The protection performed by conformal coatings can withstand higher voltage gradients and allow closer circuit track spacing, even it can defend the shock circumstance, in turn enabling electronic designers to meet the demands of miniaturisation and reliability.

What is Conformal Coating Applications in Electronics Industry.

The outstanding technology’s evolution today has resulted in electronics and devices being exposed to a range of unique environments. Therefore, some sensitive components and connections sometimes have to be exposed to environmental hazards such as moisture, corrosion, dust, and extreme temperatures. Any of these environmental conditions would take harm to the electronic components’ integrity and functionality. And for this reason, PCB assembly manufacturers normally take protective measures to secure the integrity of printed circuit boards (PCBs). One protective measure to have the PCB boards protected by conformal coating.

The universal use of conformal coatings is popularly applied in communication, automotive, military, aerospace, marine, outdoor lighting, oil&gas, industrial control, IOT devices,and green energy.

However due to the rapid development of the electronics industry, conformal coatings are also expanding their application into the domestic and mobile electronics industries, providing the necessary combination of excellent performance and reliability within a vast array of electronic devices.

Conformal coating UV light inspection

The Reference of Conformal Coating Selection:

Cost is always the first consideration when we are trying to choose the conformal coating for the printed circuit assembly board. It is important to understand that to get the right level of protection from the coating material you would have to be very carefully in selecting the right material for your electronics’ application.

Different types of coating material have different performance in different operating environments, therefore selecting the most right coating material for your application is the first step to achieve the levels of protection that you require. As a PCB assembly manufacturer, our team can provide the basic reference for you to make your selection for your design,what’s more our technical support team are always on hand to answer any question you may have and to freely assist you in selecting the right material.

The Types of Conformal Coating

Many types of conformal coating materials exist and can be categorised by their base chemistry, including a wide range of specialty varieties and each option having its own beneficial property for the different environments the Printed circuit board will be operating in. The most common of these conformal coatings along with their most prevalent applications are described below:

Urethane resin: Urethane resins (UR) also called polyurethane resins are known for its’ excellent chemical resistance and superior protection to moisture, abrasion and chemical attacks. The core feature of it’s downside of this material is that it is resistant to solvents, that means it is difficult to remove and rework once it is applied to the PCB board (only can be removed using specialist products (CCRG)). Urethane resins are most commonly used in some high aerospace applications coating where the components may be exposed to corrosive fuel vapors.

Acrylic resin: Acrylic resins (AR) are acrylic polymers which have a limited solvent resistance. These substances require a simple drying process and are easy to rework but not applications where chemical resistance is important. The coating completed by this type of material can provide excellent protection against humidity and other environmental factors but poor protection from solvents and chemical vapors.So,it is most suitable for basic-level protection. As the major upside of this material is the ease so with which it can be easily removed, reworked and quickly dried, making it practical for repair operations via simple solvent evaporation.

The coating material Weller used is Humiseal-1B73

Key features of Acrylic coatings:
Good flexibility and global protection
Have a wide operating temperature range.
Excellent protection against humidity and salt mist
Have excellent adhesion to a wide variety of substrates
Easy to apply and remove and dry quickly under room temperature by simple solvent evaporation

Epoxy resin: Epoxy resin coatings is a type of compounds that can create a hard layer for electronic PCB board with good resistance performance to humidity, abrasion, chemical, and with minimal permeability. They have less flexible features than other types of coatings, and epoxy resins are difficult to remove and rework.

Epoxy is the most ideal choice for the printed circuit boards that need to be completely coated and is often chosen for applications with low mechanical stress.

Key features of Epoxy resin:
Very Flexible
Good global protection
Ease of processing and operation
Approved to IEC 61086 & IPC-CC-830
Have Wide operating temperature rangesion

Silicone resin: Silicone resin coatings can provide excellent thermal, chemical, moisture and corrosion resistance with maintaining good flexibility. It is difficult to remove and offers poor abrasion resistance due to the surface’s feature of rubbery texture. However, this property makes it up for this in offering good resilience against vibrational stress. Silicone resin conformal coatings are often chosen for electronics production in high/low temperatures or temperature fluctuations due to their more flexible nature. But, they are less resistant to humidity and corrosive gases than other type of coating technologies.

Key features of Silicone resin:
Good flexibility and global protection
Have a wide operating temperature range.
Excellent protection against humidity and salt mist
Have excellent adhesion to a wide variety of substrates
Easy to apply and remove and dry quickly under room temperature by simple solvent evaporation

Parylene: Parylene is a type of coatings applied through chemical vapor deposition. By this process, the parylene is heated up and become a gas and put into a vacuum chamber to polymerize and turn into a type of thin film placed over electronics devices. This film offers good dielectric strength and excellent resistance to extreme temperatures, moisture and corrosive elements. However, it is difficult to remove and rework by normal operation and requiring abrasion techniques to remove it. It’s specialized production process determine that the work of replacing parylene coating more difficult than other methods. Therefore It’s often used in specialty applications.

UV Cure Coating: UV cure conformal coatings is another type of hybrid coatings. For instance , the different chemistry types are often used to generate a dual-cure system, thus avoiding issues with areas shadowed and detected from the UV light during curing. The UV cure coatings can offer the fastest curing times with coatings curing within seconds under UV light. This is highly beneficial to high throughput applications and any shadowed areas on the PCB board will cure through the secondary cure system over time. Due to the highly cross-linked structure composed when cured, so UV cure conformal coatings is difficult to remove and rework and in some cases it is known to suffer from reduced flexibility when thermal changes and therefore must be carefully tested.

Key features of UV Cure Coating:
Extremely quick process times
Excellent performance on chemical resistance
Excellent protection in a range of environments
Secondary moisture cure system used to help shadow areas cure

Conformal Coating Standards

Conformal coating standards are a series of specifications and tests criteria that a coating must follow and pass so as to be considered for use in certain environments such as Automotive or Military applications.

Most conformal coatings material in supply are either qualified to specification of MIL-I-46058C or meet the requirements of IPC-CC-830B specification. In addition they may be recognized by UL (Underwriters Laboratories) if its electronics related to a safety Compulsory application, either as a permanent coating material, in which case the flammability of the coating is assessed to UL 94V-0, or as a conformal coating, basically where the electrical properties assessed as part of the UL746E standard.
Many coating materials will have one or both of these certifications IPC-CC-830B ,UL 94V-0,or UL746E, which indicate the coating’s safety for use on PCBs:

StandardCore specification
MIL-I-46058CA common standard used throughout the industry for conformal coating materials since 1998.It is still a standard polularly required by military contractors. The main advantage of MIL-I-46058C is that it requires independent testing by a MIL approved laboratory (although at least one conformal coating manufacturer is an approved test laboratory so not completely independent!) and the military maintains a Qualified Product List (QPL) drawn from successful independent test reports.
Def Stan 59/47This is a standard used by the British military for coatings used in high end electronics,which similar to the MIL-I-46058C
IEC 61086An International Microelectronic Commission based on supplier self-certification, with its standard similar in requirements to MIL-I-46058C. Of course, an independent test laboratory or 3rd test laboratory can be used, but the IEC does not maintain a QPL.
IPC-CC-830BClosely related to MIL-I-46058C and then transitioned into a civilian version. Commonly the testing facilities refer to one or both of these standards interchangeably. It primarily focuses on the coating’s features in appearance, flexibility, flammability, stability and resistance to insulation, fungus, moisture and thermal shock. Coating Materials qualified to MIL-I-46058C are automatically deemed to meet the requirements of IPC-CC-830B.
UL 94 V-0UL94 relates to the flammability or self-extinguishing nature of the conformal coating material on an FR4 substrate. V-0 is the highest grade that can be achieved, with other categories such as V-1 and V-2 denoting materials that burn for longer durations.
UL746EUL746E tests are standardized by UL (Underwriters Laboratories), a reliable global safety certification organization that regulates safety for consumer electronics. UL746E tests specifically concentrate on the electrical and fire safety of coated electronics. Once a coating material passes UL746E, it is registered with UL under a specific number and must be retested annually to maintain registration. By documenting the use of UL recognised materials throughout the design, manufacturers can automatically avoid the requirement to test their device to the applicable UL safety standards.

In addition to above serials certifications, PCB conformal coatings must also meet basic standards for environmental and worker safety. Some of the most significant regulations that apply to PCB conformal coating in the electronic industry include guidelines from the Occupational Safety and Health Administration (OSHA), which mostly focuses on worker safety.

How Does Weller Apply a Conformal Coating?

The application method of a PCB assembly conformal coating is very important as the selection of the right coating material for your application.

This is because it will not provide the right levels of protection as required eve it is the best coating martial but applied by poor production process.

In Weller, we apply all the coating by automatic machine instead of manual work to achieve the stability and the standard thickness up to 150um.

Weller Manufacturing and Assembly Services

When it comes to choose a conformal coating for your printed circuit board assembly projects,essentially it is to follow the application’s requirements and relevant regulatory standard or specifications. If you’re looking for a PCB fabrication and assembly manufacturing company that can help you choose and apply the right conformal coating materials, Weller Technology can help.

Weller Technology is a professional electronic manufacturing company in China that offers a full range of PCB manufacturing and assembly services backed by 10+ years of experience.

By partnering with Weller Technology, you can enjoy the benefits of quality project assistance and top-of-the-line of PCB manufacturing and assembly practices, including full conformal coating services.

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