PCB Protective Coatings: The Best PCB Design and Manufacturing Practices for Moisture Management
I once had an outdoor sensor system with a custom PCB protective coating that would stop recording data for two hours at dawn every day. This was not because the board needed its beauty rest before it went to work for the day. In actuality, the sensor box was collecting the early morning condensation and causing the board to malfunction. Obviously, it never occurred to us that our PCB might be exposed to moisture when deployed in the field. While this does not sound like a good situation, we considered ourselves lucky that nothing shorted and the system resumed its normal operation once dry.
It’s rare to be that fortunate, but now I know that moisture is a very reason why PCBs malfunction. Now that you know that condensation can kill, there are several options for PCB protective coating to handle PCB moisture management.
Keep your boards dry - case them in plastic!
Put your PCBs into Enclosures
The simplest solution for PCB moisture management is to put your Printed Board in a box. Enclosures minimize changes to your PCB design and are relatively inexpensive. For a prototype, this might be as fancy as Tupperware sealed with silicone. (Protip: Clean Tupperware only. Potato chips and electronics should never mix.)
You can buy ready-made boxes and cases with a variety of sealing options in nearly any size.
If you level up and design the casing yourself, consider using rounded sides so that condensation on the walls will flow away from your PCB. Also, include slots to hold the Printed Board in place. You should design your board with enough margin to fit in the slots without touching the components.
You can also include desiccant (you know, the DO NOT EAT packets from shoe boxes and beef jerky) to absorb moisture in the box.
A sealed enclosure is often an easy solution because you’ll need to package your electronics anyway. However, in some situations, it’s just not enough. You should probably be looking at additional protection options if:
You need rigorous protection due to environmental conditions,
The enclosure will be reopened regularly
There are holes in the enclosure for wiring or other I/O considerations.
Apply a Conformal Coating
A conformal coating is the most PCB protective coating option. There are several types, but they are generally cheap to apply and can be scaled with manufacturing.
Before selecting a conformal coating material, determine if you need to protect against anything else in addition to moisture. Our systems were deployed outdoors, sometimes near agricultural areas, so we worried about temperature, inclement weather, and pesticides that might penetrate the casing (which we did have to reopen frequently). We also had some surprise fungus in one system! This turned reworking the boards into a particularly special occasion.
When choosing a conformal coating, you should consider these properties:
Moisture resistance: That’s why we’re all here. Most conformal coats will be billed as protecting against moisture. However, any air gap on the circuit board can impact the moisture protection. Not all coatings are created equal, so check your manufacturer specs to be sure it will protect against rain, humidity, or whatever conditions you’re expecting.
Chemical resistance: This should be a top consideration if pesticides, saltwater, or other corrosive chemicals might come in contact with your circuit board. Not all coatings will protect against all chemicals.
Temperature range: Does your coating need to survive large or rapid thermal changes? If yes, then your coating needs to flex with the circuit board, so it doesn’t strain or crack the components and solder.
Dielectric properties: The dielectric properties of some coatings can be both a good and bad thing. Insulation from the coating can give you more leeway in a design by reducing spacing. On the other hand, the dielectric properties of some coatings can leave you completely out of luck by changing the RF impedance matching that took weeks to optimize.
Ease of application: Who is applying the coating and how pretty do you want it? You can brush or paint the coating on, but this usually turns out to be a hot mess. I recommend spraying or dipping your boards. Alternatively, automated processes are more consistent in their application, though they’re much more expensive.
At a startup, we sprayed all the boards by hand. Front of the board in the morning, the boss takes us out to lunch, and the back of the board in the afternoon. I vouch for this schedule.
Flexibility: If you have sharp edges, lots of solder joints, or other irregular topography on the board, you need a coating that is very flexible and distributes smoothly across the board. Ask the supplier about the coating’s viscosity and flow to find out which one you need.
Rework: Coatings are intended to protect the board and not come off easily. If you anticipate any rework, pick a coating that’s easy to strip off without obscure chemicals or horrifying solvents.
No one likes to do repairs to their boards!
Most coating materials are
Silicones: These generally have a high-temperature tolerance and are easy to apply yourself. They’re flexible and resistant to a lot of chemical and moisture conditions.
Urethanes: These are great when you’re looking for hardcore toughness. This includes resistance to scratching. They usually require equally hardcore strippers if you want to remove the coating and rework the board.
Varnishes: These have a natural resin base and come in a huge range of varieties. They have an equally large range of temperature tolerance, moisture protection, and flexibility.
Acrylics: These are a good, general protection option. They also come with the added bonus that they are usually easy to remove from the circuit board, if necessary.
Additionally, make sure to mask any components that shouldn’t be coated before you start protecting your boards.
Pot Your Electronics
Sometimes you have to go all-in on protection, which means you’re looking at potting. This entails embedding the entire printed circuit board in a protective material, usually an epoxy or silicone.
Potting is usually done at the component level, but Printed Boards in a marine or other electronically brutal environment may need the entire circuit board protected. When you scale up, you’ll need a manufacturer to do this. Voids or air gaps in the material are difficult to avoid and will undo all the work you’ve done to protect your board.
Temperature requirements are an important consideration. Once the whole board is embedded in epoxy, the compression and expansion caused by temperature changes can strain solder joints and packaged components. If the strain causes cracks then you’re going to have a bad time; it’s really difficult to rework your board after it's been damaged. Companies like Epic Resins and Master Bond have options with high thermal conductivity and low thermal expansion to help prevent these kinds of issues.
Make it easy on yourself!
When you design for manufacturing, printed circuit board protection is as important as component selection. Otherwise, you’ll find yourself like me, slogging through a marsh at dawn to do manual measurements while your board suns itself back into operation.
To keep track of the specs of my layout, I’ve made massive spreadsheets containing their pitch and temperature deratings. It is terrible. Keeping it up-to-date is almost as bad as retrieving your malfunctioning Printed Board from a buggy swamp at dusk. As soon as you slap away each blood-sucking annoyance, another replaces it. A painless solution is to use professional PCB design software like Altium Designer®, where you can incorporate the specs for your components into embedded design rules. More so, their add-on will help you keep track of your component specifications and verify whether they are appropriate for a specific PCB conformal coating or epoxy. Altium can also check the 3D clearance for your enclosure as you design. Using these tools, you can design your PCBs successfully and with ease, no slogging required.
For more information about how your PCB Design software can help you, contact an expert at Altium Designer.