Electrical Hardening Techniques for Outdoor Printed Circuit Boards
February 26, 2018
Printed Circuit Boards (PCBs), like the one pictured above, connect and support electronic elements using conductive tracks, pads, and other features etched (typically from copper) into a non-conductive substrate (generally FR-4 glass epoxy). Because the substrate and electrical tracks are sensitive to environmental changes, special consideration must be made for PCBs used in outdoor environments.

PCBs used outdoors typically have enclosures to protect them from the elements. When designing PCBs rated for outdoor use with an enclosure, many factors need to be considered to protect the PCB. These include, but are not limited to, temperature, dust particles, vibrations, electrical interferences, and many other environmental factors such as rain, snow, hail, and vermin. The design of the PCB and the enclosures that house them must bare these conditions in mind.

This article will explore some guidelines, testing techniques, and potential methods to help protect PCBs from external environmental elements. These guidelines and techniques are gleaned from a number of standards bodies including the American Society for Testing and Materials (ASTM), the International Electrotechnical Commission (IEC), the Institute for Electrical and Electronics Engineers (IEEE), the German standards body (DIN), and the Verband der Elektrotechnik, Elektronik und Informationstechnik (VDE). This is not an exhaustive list of techniques but one drawn from the experience of the authors in the manufacturing of low and medium voltage (up to 38kV) power equipment. These methods are used to improve the dielectric strength of equipment 1,2 minimize electrical tracking,3 and improve transient hardening in PCBs.4,5

Environmentally Sealed Enclosures and Outdoor-rated Equipment

Outdoor-rated equipment must be rated to specific standards and environmental seals. One enclosure rating is the International Protection (IP) Marking. An IP rating of IP68, for example, indicates a “6” rating for dust of “no ingress of dust; complete protection against contact (dust tight) following an 8-hour test”; it also indicates an “8” rating of water resistance to being underwater for a duration and depth agreed upon by manufacturer and user.6    

The National Electrical Manufacturers Association (NEMA) also rates enclosures for their environmental capabilities. For example, to earn a NEMA 4X watertight rating, an enclosure must exclude at least 65 gallons per minute of water from a 1-inch nozzle delivered from a distance not less than 10 ft. for 5 min.7  The “X” indicates additional corrosion resistance such as the use of stainless steel.

Another issue to consider is the specific plating material of terminal connections (Nickel vs. tin) in an effort to avoid corrosion of the terminals due to moisture ingress if an environmental seal is broken. Corrosion of the terminals may render the connection unusable, which can then prevent the PCB circuit from operating as intended. In addition to moisture, the terminal plating material should also be able to hold up to the harsh external environment, such as a sulfur or fluoride environment, if necessary. When designing and manufacturing equipment for outdoor use, wire insulation for desired application must also be considered, including things like offshore use and flame retardancy (ex. No smoke zero halogen wires), among others.

Dielectric Strength Improvements and Electrical Tracking Reduction Techniques

To avoid insulation breakdown, one must space out the PCB tracings to exceed the desired dielectric strength. If a physical space increase is not possible, one may use an insulating material to increase the dielectric strength between two traces or between an energized component and ground on the PCB. For direct outdoor use, the PCB should be placed in an enclosure and encompassed in an insulating material. The insulating material may include an insulating shield and/or liquid between the energized PCB and ground reference. An example is a silicon adhesive like RTV-11 or an epoxy that fills in the crevices between the board and ground reference and then cures/solidifies after a set period. Not only does this improve dielectric capability but it also helps prevent moisture, dust, and other environmental particles from entering the housing, which can then lead to failures.

For air gaps, there are a few options for PCBs and PCB enclosures used outdoors. In some cases, powder coatings can be used to improve dielectric and electrical tracking strength in a tight air gap. Another option is to use a sealant or gasket to seal the gap between the enclosure and the lid to prevent ingress of water and other particles from the outside environment.8  A third option is to remove the air from the insulating material by pulling a vacuum on the liquid insulating material before it cures. Degassing may also be required depending on the process material used and the desired result. Additionally, there is the option of using a specific type of wire (ex. Litz wire) that permits impregnation of the insulation jacket, though this material must be viscous enough to flow inside the insulation jacket and fill the voids for this method to be effective. Additional information on dielectric strength, creepage, and clearances can be found in a prior article in addition to standards.9

Testing Methods

Methods for testing outdoor-rated equipment worth noting include Thermocycling10  and Electromagnetic Shielding11. Thermocycling is the process of exposing a piece of equipment to the minimum and maximum temperatures it needs to endure. In some cases, one may want to conduct additional testing by physically damaging the test subject (making holes, cracks, dents) after thermocycling a brand new unit, and then performing the thermocycle test again for minimum and maximum temperatures. This process will allow one to observe (if any) progression of damages on the test subject, which emulates long-term effects if the test subject was damaged either intentionally or by force majeure. One may need to repeat these tests for multiple thermocycles and/or multiple test subjects to truly assess potential environmental damage. It is also advised to exceed the minimum and maximum temperatures the test subject is rated for by 5–10°C to provide some margin.

Next, to address issues of electromagnetic interference, it is possible to enclose the PCB with an electromagnetic shield to minimize unwanted external magnetic fields from the outside environment, as per IEEE Standard 299. Further testing can be performed for electrical transients hardening by subjecting the PCB inputs to numerous voltage waveforms (sinusoidal, square, and triangular) at various frequencies and voltage magnitudes beyond what industry standards require. Two electrical transient hardening techniques involve decoupling and analog filtering. One can decouple the input, output, and any intermediate electronics from each other on the PCB to avoid transferring the damaging electrical wave between components. Analog Filtering (with high pass filters, low pass filters, bandpass filters, etc.) will minimize the transient effects on the PCB from unforeseen system anomalies.


  1. ASTM D149
  2. IEEE Std. 4-2013
  3. IEC 60112
  4. UL 1283
  5. AN80994 – Design Considerations for Electrical Fast Transient (EFT) Immunity. Cypress. http://www.cypress.com/file/138636/download
  6. http://www.certifigroup.com/whitepapers/IPCodewhite.pdf
  7. http://www.paramountlighting.com/resources/pdf/NEMA-4X.pdf
  8. ASTM D3638
  9. S. Phan, J. Stepan, B. Cotts, “Electrical Conductor Spacing Standards for Printed Circuit Boards,” Exponent Electrical Engineering and Computer Science Newsletter, Volume 4, 2016.
  10. ASTM D7895
  11. IEEE 299