Let’s say I have two, identical circuit boards. Each board has an acrylic conformal coating applied to it. The coating was applied to protect the board material and electronic components from typical levels of humidify and occasional condensation. Let’s also say the thickness of the coating is around 25 μm.
One circuit board is constantly exposed to air that is saturated with water vapor (RH ≈ 100%) for a few weeks. The other circuit board is completely immersed in liquid water for a few weeks. Each at a temperature a little above room temperature.
In each case, obviously, water molecules from the environment will go through the coating. This could eventually cause serious problems with the board (short circuits, corrosion, etc.).
But here’s my question: which circuit board will receive the most damage due to moisture? The one that is exposed to air that is saturated with water vapor, or the one that is completely dunked in water? And why?
I’ve been doing lots of google searching on this, but haven’t really found much on the topic. I’m not a ChemE, so some of the papers I’ve found are indecipherable to me; lots of talk about wetting angle, surface energies, dissolved oxygen content, etc.
My gut intuition is that a board (with conformal coating) that is completely dunked in water will be damaged more quickly than one that is exposed to air at 100% RH. I just wished I had some actual science to back it up.
My “educated” guess is that 100% RH would be worse, due to the presence of all the oxygen in the air. But, RH is useless without a temperature. 100% RH at 0C is safe, not so 100% RH at 95C, which is exceptionally corrosive.
I agree. For certain military and aircraft electronics the ultimate durability test is the Salt Spray test which is done in a cabinet in ambient atmosphere. Submersion in liquid generally suppresses corrosion to a degree.
Would it be true that there are more molecules of water per square inch making contact with the surface of the board dunked in water versus the board at 100% RH? If so, how do I calculate the density of each (# of water molecules/in[sup]2[/sup])?
The number density of pure liquid water is about 55.5 moles/L. You can use that, and Avagadro’s Number, and the definition of a liter, to find the number of molecules per square mm or whatever. If you want the number density better, you can start from the mass density of water at the temperature in question, then use the molar mass of water to convert from mass density to number density.
To do the same for humid air, you need to look up the vapor pressure of water at the known temperature and total pressure. Then you can use the ideal gas law (which is good enough) to convert from the partial pressure of water vapor to the number density of water vapor. Then you’re back to what you did in the first paragraph.
You will certainly find the areal density of water is far higher in the liquid than in saturated humid air, which means there will be many more collisions per second of water molecules with your surface in the liquid. If you are looking for only penetration by water molecules, and damage thereby, and want to set aside any question of damage caused by other things – e.g. including electrolysis – that may be assisted by the presence of water, then I’d say your answer is clearly the liquid, for the simple reason that it will penetrate faster.