I can remember a time when I ignored a rule set down by my mother about wearing a coat on a winter day. I was in grade school at the time, and way too cool to be fettered by the constraints of my styleless coat, so I snuck out of the house without it. The problem though was that it was cold and rainy outside, I was only wearing a T-shirt, and I had to walk 20 minutes to get to school. I’m not sure if I actually got sick or not, but I definitely suffered some discomfort by not following the standards set up for me for which conditions required a coat.
Printed circuit boards also need protection from the different environments that they operate in, and that protection comes from conformal coating. Fortunately, we have some guidelines to help us determine how this coating should be used. These guidelines are available in the standards written for conformal coating, and they cover a lot of information about the coatings chemical makeup, application, and intended use. Let’s take a closer look at IPC conformal standards as well as some other information on how to best protect a circuit board.
What Are the Different Types of PCB Conformal Coatings, and What Are They Used For?
For protection against the harsh abuse of their operating environments, printed circuit boards are often covered with a conformal coating. This coating is a thin non-conductive dielectric film that can be brushed on, sprayed on, or applied by automated processes that include spraying or dipping. It is designed to protect the components of a circuit board as well as solder joints, and other exposed areas of metal. It will even help strengthen the board against thermal and mechanical stress, as well as rough handling.
Conformal coatings are classified by type according to their chemical makeup:
AR - Acrylic resin: This coating is easily applied and removed, and is easy to work with plus it is affordable. It has, however, a low resistance to chemicals, solvents, and abrasion, and is not the best for high temperature and harsh environments.
UR - Urethane resin: This coating has great chemical, humidity, and mechanical wear resistance, but it suffers from long cure times with a risk of peeling. It is also difficult to remove.
ER - Epoxy resin: This coating has great abrasion, moisture, chemical, and humidity resistance and provides excellent protection in harsh environments. That same protection, however, makes it difficult to remove, and it may shrink during curing.
SR - Silicone resin: This coating performs well in extreme temperatures, and has excellent humidity and corrosion resistance and good chemical resistance. It also adheres well to the majority of PCB materials and components. However, it is the most difficult coating to remove requiring chemical solvents or aggressive abrasion.
XY - Parylene: This coating has the best resistance to extreme temperatures and solvents, plus it has a high dielectric strength and is transparent. The coating forms at room temperature and doesn’t require any time for curing, but it is difficult to remove and must be scraped off. It also requires chemical vapor deposition equipment, and it is not rated well for outdoor exposures of any length.
Initially, conformal coatings were expensive and were usually restricted to mission critical military, aerospace, automotive, marine, and industrial uses. Now as the conformal coatings become less expensive, they are beginning to be used more for common consumer level electronic products. The next thing that we will look at is the standards that cover conformal coatings.
Spraying on conformal coating using a programmable selective dispensing system
What Are the IPC Conformal Coating and Other Standards?
The Association Connecting Electronics Industries organization, or as it is more commonly known; “IPC,” has some standards that govern the capabilities and requirements of conformal coatings:
IPC-CC-830C: This is the main standard for conformal coating. It was actually created to provide an alternative to the older MIL-I-46058C standard when it was deactivated. 830C is used to qualify different conformal coating products, and contains sections on materials, shelf life, curing, chemical, viscosity, and appearance requirements. It also contains standards on fungus, moisture, and flammability resistance as well as dielectric withstanding voltage and other electrical specifications. This standard is an active specification, and is updated with new information as needed. The latest “C” release was published in January of 2019.
IPC-A-610G: This standard covers the acceptability of electronics assemblies, and as such has a section on the requirements for conformal coating coverage and thickness.
IPC J-STD-001F: This standard covers the requirements for soldered electrical and electronic assemblies, and has a section dealing with materials and application of conformal coating.
There are also some other standards that include specification and requirements for conformal coating:
MIL-I-46058C: This is the original military grade standard for conformal coating. 46058 was deactivated in 1998 for new designs, and IPC-CC-830 was created to replace it. Since the two specifications are similar, conformal coatings that have been qualified to 46058 are considered to have met the requirements of 830. In spite of its deactivation however, it is still a standard that is referenced and some military contractors are still required to meet it.
UL94: This is an Underwriters Laboratory standard that certifies the ability of a conformal coating to self-extinguish a flame on a circuit board substrate (FR-4).
UL746E: This is another UL standard that covers the test procedures for the physical, electrical, flammability, thermal, and other properties of the materials, including conformal coating, used in printed circuit boards.
Now that we know which standards to look at, what is the next step?
Circuit boards can get protection from harsh operating environments with conformal coating
Conformal Coating and the PCB Designer
To successfully design a printed circuit board that will be conformally coated, you should start first by familiarizing yourself with the standards. This will give you a foundation of understanding that will help you through the next step, talking with your PCB contract manufacturer. Your CM will be able to help you in understanding which conformal coating is going to best fit your needs, and how you should design for it. Once you are armed with this knowledge, you will be ready to design the board and create the manufacturing drawings needed to have your board coated.
To be able to design a board and create good manufacturing drawings, you need a PCB design system that has the power and capabilities to do the job well. OrCAD PCB Designer from Cadence has the tools and functionality you will need to not only capture a schematic and lay out a printed circuit board, but also to create detailed and accurate manufacturing drawings. You will be able to specify to your manufacturer what you need for conformal coating so that there won’t be any confusion or mistakes along the way.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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