I am facing challenges with one of my PCBs, which is exposed to moisture in outdoor environments. The problem is that moisture keeps finding its way into the enclosures, sometimes leaving the circuit boards exposed to water for hours at temperatures exceeding 100°F.
I’ve noticed corrosion developing on the solder joints. Recently, moisture got under the solder mask and completely ate through a trace, causing a circuit break. While a perfectly sealed enclosure is the ideal solution, I’d like to add additional protection to the PCBs as a secondary defense.
Are there any tips or tricks for making PCBs more resistant to corrosion in these harsh outdoor conditions? Would removing excess flux be beneficial? What about applying spray acrylic sealant or using special solder?
This is something assembly handles but you should look at encapsulation/conformal coating. Basically the spray sealant you mentioned. There are different materials out there so you should make sure to get one that best fits your requirement.
Our assembly manager says:
- Prior to coating the board, the board has to be washed and baked, the baking time, and temperature depends on the designer of the board.
- Baking is to dry out all the moisture in the PCB.
- Once the coating is done, assuming 100% coating on board. It needs to be inspected under UV light to see if there is any bubble or moisture still trap inside of the board.
The coating solution suggested by Steve Carney is indeed the most effective option.
Another crucial consideration is minimizing DC voltage gradients wherever feasible.
To elaborate, when selecting pins on components, avoid choosing adjacent pins where one consistently carries a high voltage, like +5V, while the other remains at 0V (GND).
This constant voltage gradient between adjacent pins can lead to electrochemical reactions, corrosion, and related issues.
Separate such pins whenever possible, opt for lower voltage levels, and consider modulating them as a last resort if signal modulation is feasible, even if it serves no functional purpose. For instance, unnecessarily pulsing an LED or switch can help avoid forming constant voltage gradients.
Regarding conformal coating on PCBs, I came across an unconventional approach from a garage shop. They admitted to simply dipping the boards in urethane deck varnish and claimed this worked well enough for PCBs intended for saltwater marine installations. They say the cosmetic appearance wasn’t great, but the overall coverage was complete.
For hobby projects or garage setups, experimenting with this approach could be worthwhile. The main challenge with conformal coating is proper masking to prevent coating from reaching unwanted areas. Most coatings are transparent and difficult to inspect (UV dyes often don’t distribute evenly), so precise application is crucial. Dipping offers an advantage in keeping connectors clear if they’re on one side, or you can use an acid brush to carefully paint around them.
In summary, while not a proven method, using a readily available urethane deck varnish might be an option to consider if you’re working on an outdoor project and need a budget-friendly conformal coating solution.
Conformal coating, particularly silicone-based coatings, may be a less invasive solution for protecting PCBs. However, if your component is particularly small or exposed to a harsh environment, potting might be considered. Keep in mind that potting makes boards extremely difficult to rework if needed, whereas conformal coating, although somewhat tedious to remove, allows for easier rework if necessary.