Analyzing PCB Coatings

PCB coatings are a thin film of material applied to a completed circuit board that “coats” the board and its assembled parts with a protective substance for additional protection. When first introduced, these coatings were initially used only on circuit boards exposed to extreme environmental conditions due to the expense. The circuit boards in these categories included industrial uses, marine/automotive applications, and military/aerospace electronics.

However, with the cost of their application dropping, conformal coatings are now used to protect circuit boards operating in environments that aren’t always as extreme, like wearables. With the dense circuitry of wearable electronics and the challenging conditions they have to operate in, including vibration, moisture, and shock, PCB coatings have become necessary to ensure their uninterrupted performance. This definitive guide will examine PCB conformal coating and its application benefits.

Why Conformal Coating?

Conformal coatings are a thin film of protective material that covers or “coats” an assembled circuit board. Coatings are made from non-conductive materials and can cover part or all of a PCB. Conformal coatings will cover areas of metal that generally aren’t covered with solder mask, giving those areas additional protection from their operating environment. These areas include exposed traces, leads, other metal on components, and solder joints. Conformal coatings will also help protect a circuit board from stresses, rough treatment, chemicals, and corrosion while protecting the components.

Conformal coating will primarily protect a circuit board from the following:

Condensation and moisture
Dust and dirt
Fungus
Salt-spray

Conformal coatings protect the board from mechanical stress from regular device usage or rough handling and help manage stress from changing thermal conditions. They are also resistant to many different chemicals and solvents. Finally, the conformal coating protects the circuit board by sealing delicate components and preventing leakage and potential corrosion.

Applications for PCB Coatings

Both conformal coating and encapsulation provide thermal, moisture, chemical resistance, and electrical insulation, and the difference is in how much protection they provide. While the conformal coating is a thin covering over the circuit board, an encapsulation or “potting” surrounds and seals the PCB instead of merely coating it. A casing or “pot” is built around the PCB to encapsulate a circuit board, and the encapsulating fluid is poured in, completely covering the circuit board with a thick protective shell. Here are some of the differences between conformal coating and encapsulation you should be aware of:

Conformal coating	Encapsulation
A thinner coating supports flexibility Usually transparent Minimal PCB weight increase Less expensive Easier application	Thicker material is better protection Darker colors hide proprietary circuits Large PCB weight increase More expensive Additional process steps

While encapsulation has its detractors, such as its expense, weight, and increased process steps, it provides greater protection for those circuit boards subjected to extreme abuse.

Conformal Coating Application Process

Conformal coatings are usually between one and five mils thick (25 to 127 microns) to provide the best protection possible while minimizing the amount of circuit board operational heat trapped within the coating. There are several methods for applying a conformal coating, with the following being the most common:

Automated spraying – PCB production runs often employ automated spraying, in which reciprocating spray heads apply a consistent coating to the boards as they proceed through the process on a converter belt.
Selective automated spraying – This process is very similar to automated spraying, except programmable sprayer heads are used instead of reciprocating. With this level of control, it’s possible to apply coating to specific areas of the board.
Manual spraying – Using spray guns or aerosol cans, technicians can quickly apply conformal coatings to low-volume runs of circuit boards. Manual spraying is also ideal when masking areas of a board to restrict the application of the coating for unique requirements.
Dipping – The standard method of applying a conformal coating to circuit boards in high-volume production runs to coat both sides of the board quickly.
Brushing – A slow procedure used during rework or repairs of a circuit board with its conformal coating stripped away for the work.

Conformal coating application often uses a liquid. It is important not to apply conformal coatings that are too thick as they will increase the curing time and potentially lead to problems such as stress shrinkage and cracking. For the best results and maximum protection, conformal coatings should follow the thickness stated in their specifications. Not only will this give better results, but it will also be less wasteful for the coating material. It is better to use multiple thin coatings for those that require a thicker application instead of applying them all at once.

Conformal coatings require a curing period. Some coatings will cure through the evaporation of their liquid carrier, leaving behind the coating resin, although it may take days to cure fully. Other coatings can accelerate their curing with heat or UV, and in many cases, a combination of methods completely cure the coating around all of the components on the circuit board.

Industry Standards Governing Conformal Coating Applications

Many protective materials are available for electronics and engineered conformal coatings explicitly designed for printed circuit board applications. As you would expect, there are also different manufacturer and user specifications detailing the use of these products. However, there are specific industry standards that apply directly to the use and application of conformal coatings for circuit boards:

IPC-CC-830C– Replacing the deactivated MIL-I-46058C, 830C is the primary standard for conformal coating of printed circuit boards. This standard contains sections on materials, curing, shelf life, viscosity, chemical, and appearance requirements and can be used to qualify conformal coating products. 830C also contains specifications on moisture, fungus, flammability resistance, and electrical specifications such as dielectric withstanding voltage.
IPC-A-610G– This standard contains conformal coating coverage and thickness requirements as part of its acceptability of electronic assemblies.
IPC J-STD-001H– This standard requires soldered electrical and electronic assemblies. It includes a section detailing the application, materials, thickness, and uniformity of conformal coating and encapsulation.

Designers should investigate other industry standards, such as UL94 and UL746. These two standards cover the ability of conformal coatings to self-extinguish flames on a circuit board and the test procedures for materials such as conformal coating used in PCBs.

Conformal Coating and PCB Design: What You Need to Know

The first step in designing a circuit board for conformal coating is to familiarize yourself with the requirements and the applicable standards. We’ve already listed the most common industry standards; you may find more specific specifications in the coatings you decide to use. It is best to work ahead with your PCB contract manufacturer to understand the standards fully and ensure compliance with the proper requirements. Here at VSE, we can advise you on your options and recommend how best to lay out your board for the conformal coating you are using.

Here are some general PCB layout recommendations to keep in mind as you design a circuit board that requires conformal coating:

It is helpful to simplify the coating application to place components that don’t require coating on one board edge.
Tall components may shadow smaller components during the conformal coating application, and components that require coating need additional clearance to these larger parts.
The dense placement of discrete components can also make applying conformal coating challenging. Due to capillary forces, the liquid tends to collect in these areas, and conformal coatings are more likely to delaminate and crack if applied thicker than their specifications.

Conformal Coating at VSE

Choosing the PCB coating involves understanding your board’s needs and what kind of coatings will provide the protection you seek. For instance, the type of conformal coating you choose will depend partly on your board’s operating temperature. Temperatures greater than 160℃ would suggest using silicone instead of polyurethane coating. We at VSE understand the concerns surrounding conformal coating application on circuit boards. We can help you choose the right coating and show you how to prepare your design to best work with it. Our technicians work with conformal coating during rework and apply replacement coatings manually. For boards that require a production application of a conformal coating, we will work with you to choose the best vendor for the job.

If you are looking for a CM that prides itself on its care and attention to detail to ensure that each PCB assembly is built to the highest standards, look no further than VSE. Contact us today to learn more about partnering with us for your next project.