potting? silicone? epoxy coating? specifics on weather proofing Arduino circuit

Hello,
Doing a search, I see people have suggested using potting compound, silicon, 2-part epoxy, and maybe hot glue for weather proofing outdoor arduino installations. What’s a cheap option?

The epoxy’s at the hardware store are kind of expensive.

Can I use a $3 silicone caulk? Is that what people mean by “silicon”?

Would hot glue damage the circuit because of the heat when applying it?

*** EDIT ****
Right, I forgot to mention the conditions. This will be in Wisconsin - cold, subzero winter conditions, fairly hot summers, morning condensation. It’s going under the eves, protected from rain for the most part.

Unless you live in a permanently cold climate, I would NOT use hot glue outdoors, where it might be subject to hot temps (i.e. while the hot glue may protect you from rain, if it gets hot enough, it will melt again).

It would have to get pretty damn hot to melt hot glue. It would need to be on bare metal exposed to the sun in the desert or something.

Bath sealer might not weather well if it's in direct sunlight.

It would be good to know the type of climate before answering, "outdoors" doesn't tell anybody much. Also: How long does it have to last, how many wires go in/out, are there any external sensors, stuff like that....?

If you want cheap-ass I say get a small tupperware box, glue the arduino to the base. Seal the lid on with more hot glue and flip it over so the lid is at the bottom. Glue some stuff on top of it to shade it from the sun (eg. pieces of wood) and mount it with the Arduino side facing North. Maybe you could use two tupperware boxes, one inside the other. That's about as cheap-ass as I can think of.

Not all silicones are the same. If it smells like vinegar, it will corrode your electronics in a short time.

Automotive silicones are the kind that won't corrode metals.

A big issue with any hard potting compounds like epoxy and some hot glues (especially at low temperatures) is differential expansion. It can crack solder joints and PCBs.

Keep in mind that a wire can become a path for water back into the circuit. Not just around where it goes to the board, but inside the insulation.

Normal silicone is acid, it will harm the electronics for sure.

I tried using ms-polymer sealant, but it is very hard to do that without creating air pockets.

You can always use some kind of circuit board spray. I use flux spray, it puts on a nice layer on the circuit board and helps soldering.

There are actually multiple choices for standard PCB conformal coatings from MG Chemicals.

Example: http://www.amazon.com/MG-Chemicals-Silicone-Conformal-Coating/dp/B004SPJOKK

You probably could coat the arduino with the liquid silicone stuff that you coat the handles of tools with. Or get some latex paint and keep dipping the board in it until it has a thick coat.

I think the tool dip/spray is called PlastiDip

Latex paint is water based. Doesn't seem like a good idea.

I don't think Plastidip is silicone. If it were, it would be catalyzed by merely being opened and you wouldn't get much out of a container of it. It seems more like something dissolved in a solvent.

Hi , you need what is called conformal coating.

http://en.wikipedia.org/wiki/Conformal_coating

It is available as a spray or tin, you spray a couple of layers over your circuit, or paint it on with a brush.

http://australia.rs-online.com/web/c/?searchTerm=conformal+coating&cm_mmc=AU-PPC-0411-_-google-_-1_MRO_LCHC-_-conformal%20coating_Exact

Try other suppliers to get a good price.

Most have a die in them that glows in UV light so you can see if you coverage is uniform.

Tom...... :)

arusr: Would hot glue damage the circuit because of the heat when applying it?

No.

Hot glue isn't really very hot. I've got some on my hands a couple of times and much I prefer hot glue over solder.

I find them about the same. You might be experiencing a heat transfer effect. I am sure the hot melt is a better thermal insulator than solder.

fungus: It would have to get pretty damn hot to melt hot glue. It would need to be on bare metal exposed to the sun in the desert or something.

Not at all, it melts quite readily inside a car in summer even in the UK with no black surfaces or metal in contact. In a truly hot climate it is going to be useless for outdoors.

While on the subject, the difference between silicon and silicone:

"silicon", element 28 is a semiconducting crystalline semi-metallic element.

"silicone", a class of silicon-oxygen backbone polymers having extreme temperature and chemical resistance compared to organic polymers (carbon backbone).

I don't know the electrical properties of clear Krylon spray paint, but if it is non conductive, the board might be sprayed with that to provide a thin non conductive coating, which might be followed by a thicker protective product. A "floating" arduino pin might make for a way to test the paint for conductivity.

You might be able to float the whole thing in mineral oil to keep out the moisture... ;)

There's also fingernail polish, if you prefer that to Type UR (urethane) conformal coat which is what industry uses.

CrossRoads: There's also fingernail polish, if you prefer that to Type UR (urethane) conformal coat which is what industry uses.

You can get it in pretty colours, too! :)

Hi, nail polish will crack and not do a good job, you can use it to seal nuts on threads, but conformal spray is the best, you can even solder through it and just re-coat the affected area if needed.

Tom....... :)

When you talking people with experience they tell you the only way how to make electronics in a box weather/moisture proof is to drill a small hole (ie 3mm dia) into the box (at the very bottom side, of course). Everything else does not work in real life :)

Yes, without the hole, the box will still tend to breath as it goes from cold to hot. It is very difficult to truly seal the average project box so it is air and water tight under pressure.

A hole in the bottom and conformal coating would be my choice.