Moisture and voltage do not mix. When there is moisture in a printed circuit board, there is an open invitation to corrosion, and corrosion causes PCBs to fail.
In the world of perfect electronic processes, a current of metal ions passes unimpeded from the anode to the cathode. When you throw moisture into the mix, things get complicated. Moisture introduces more chemicals that lead to unwanted reactions as the ions travel from anode to cathode. These unintended reactions from moisture-induced chemicals are called “electrochemical migrations,” and they lead to corrosion.
Fortunately, you can minimize the chances for corrosion in the PCB design stage. You can do this by choosing the best layout and materials. Here are a few tips for keeping moisture out of PCBs.
- Avoid angles. Sharp points and hard angles on boards lead to higher electric fields. Opt for oval pads instead of square ones to minimize the electric fields that hasten corrosion.
- Separate high voltages. As electronic devices increase in capability but decrease in size, we end up with high voltages in close proximity. High-voltage circuits are more sensitive to moisture. Avoid the close placement of traces with major voltage discrepancies. This could lead to corrosion.
- Use grounded guard rings. High impedance circuits tend to fail first in the presence of moisture. A ring at a ground potential can minimize electric fields around high-impedance nodes and keep corrosion at bay.
- Avoid dissimilar metals. Accidental batteries tend to form in PCBs, contributing to corrosion. Accidental batteries are hastened by the use of dissimilar metals. You won’t have a problem if you stick with metals like tin, silver, lead, and nickel because they are all on par with copper in their electrode potential. Aluminum is significantly lower than copper and gold is significant higher. Thus, if you put an aluminum component on a board that is plated in gold, you could create an unintended battery. This battery would produce so much voltage that it would continue to generate even after the power supply is shut down. The power supply creates heat that can minimize humidity. With no power to bring down moisture levels, you get corrosion.
- Use caution with silver. Silver tends to form “dendrites” on PCBs in reaction to humidity in the air. Dendrites catalyze quick-spreading corrosion. If you opt to use silver, try dissolving it in solder. Also note that silver creates unwanted reactions when it comes into contact with sulfur compounds that are common to air pollution.