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Alan,
My comments about spray have to do with vent holes only. No spray, no problem. If it is very warm inside when you bring the unit outside, you may get condensation. A vented box will equalize after a while.
Rain? What is rain? I live in Arizona. Can't help on wood treatment, the termites ate all of ours years ago.
BTW: Rainwater is not the same a distilled water. It contains whatever soluble contaminants that are present in the air, i.e. acid rain.
The polyurethane coating that I refer to is not a polish, it is a finish, like a varnish. Here is a link to the type of product that I am talking about. I offer this as an example, I have not used this particular material:
Leaded solder contains tin, but a lead content of 3% or greater eliminates the whiskers. You should be able to find that info on the NASA website. This is the problem with the ROHS directive being in absolutes. A mere pinch of lead would do away with the whisker problem in it's entirety.
I am not familiar with the Liquid Tin product, but if it leaves any elemental tin (not oxide), then it can be a source of whiskers. I suspect that this is the same thing that the industry calls immersion tin. That process is a chemical replacement where the liquid is a tin salt, and the reaction exchanges the copper for tin, leaving elemental tin on the surface, with the copper going into solution as a copper salt. Clean copper quickly oxidizes, making soldering to the board difficult to impossible. The purpose of immersion tin is to keep the surface solderable when the board is not immediately used. It is an identified source of whiskers.http://www.epectec.com/articles/pcb-surface-finish-advantages-and-disadvantages.html
Thanks, Brad. I watched the NASA video.
No, Harvey A., my circuit has no glass diodes, or other components that I know to contain glass.
I am glad you mentioned condensation, as I had never thought of
it occuring on my circuit board. Having just defrosted my freezer yester
day, I know it grows quickly on cold surfaces. Since my circuit con-
troller will always be outdoors while in use, I will keep it inside when
not in use, so that condensation doesn't happen. Nonetheless, I will inspect the PCB for condensation when the outdoor temperature drops. The only "spray" the box might be subjected to is rain, which is distilled water. The box will be on my screened-in wooden porch. The porch was built with pressure-treated wood many years ago, but for the past several years moss has been growing on the wood, which should tell you how damp it is. (Another project--re-waterproofing the wooden porch.! Will probably need another recommendation for treating that wood too, but email me privately for that.)
Okay, copper oxidation won't degrade the circuit's performance.
What about tin oxide? Does either Liquid-Tin coating or leaded
solder contain tin? If yes, do you think tin oxide will degrade the circuit's
performance ? Or is too "self-limiting" and of no consequence?
I am not sure what you meant by the "Polyurethane wood polish" you recommend. I am trying to identify a furniture polish containing polyurethane. So far, I am only finding furniture polishes designed for wood furniture that has a top coat of polyurethane, like the one on your local tavern's wooden bar top. I'm not sure whether to look for a furniture polish that was designed to be applied to a polyurethane top coat, or that actually contains polyurethane as listed in the ingredients. Or are they one and the same? As you can tell, I am pretty much ignorant about this product. I am guessing its purpose is to fill in surface scratches on a polyurethane top coat.
73 to all.