Homebrew PCBs

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:

http://www.lowes.com/pd/Minwax-Fast-Drying-Polyurethane-Semi-Gloss-Base-11-5-fl-oz-Polyurethane/999913669

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

I hope this answers all your questions.

Harvey
On 7/31/2016 9:48 AM, alan00463@... [Homebrew_PCBs] wrote:
>
> 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.
>
>