PCB design for outdoor use without coating

Question

I am making a PCB for a home project: a small weather station that I will place outdoors to collect data. There is now a question of environmental resistance.

I can mill out the PCB for free on a relatively old machine. This means that I will not get any protection against the elements out of the box.

My question then is: how quickly will the PCB degrade when left outdoors (in a housing) with only tin covering the copper and are there any relatively cheap methods of protecting the PCB against the elements?

Answer 1

If your project is exposed to large variations in temperature and humidity, you may begin to see serious degradation in just a single season. It only takes a few degrees of difference between moist air temperature and your board to get moisture condensing on the board. Soldered/tinned connections degrade surprisingly quickly. If you want to test this out get a moisture detector PCB from Sparkfun and see how long it takes to corrode. Even in a non direct wet environment outside it's a problem after just a month.

Potting or deep conformal coating is the only way to get real longevity (many years).

Unless you are in a gland protected moisture proof box the ends of your cables (copper) will degrade over time. It's unlikely that you have significant current involved, but even at lower currents you cannot solder the ends of wires without long-term corrosion problems.

It is best to use solid copper cables and IDC or crimp the connections rather than use soldered or screw terminals (perhaps start reading here).

there are plenty of options for IDC connections and these are particularly effective. AVX have a broad range including SMT parts, and my personal favorite is Phoenix PCB IDC which does not require a pushdown tool and can handle both solid and multistrand wire.

For conformal coatings you can't beat MG 422 IMO, but there may be other good ones out there. MG 422 is easily peeled back for repairs, which is why I like it.

Answer 2

Buy a cheap spray can of acrylic protective lacquer for PCBs (eg https://www.electrolube.com/products/conformal-coatings/apl/acrylic/ ) , and spray both sides of the board. If you protect the coating from UV and mechanical stress, it should last a very long time.

You are presumably going to place this board in a mostly water tight enclosure?

Other possibilities are potting your circuit, either in the traditional epoxy, or one of the gel based products like raytech's magic gel ( http://www.raytech.it/product/low-voltage/magic-gel )

Answer 3

On the cheap...buy a small can of polyurethane and after assembly, dip the whole board in it. Then let it thoroughly dry and maybe do a second or third coat if needed. I made a pin stand for this where a took straight pins (the T-type) and ran them through some thin foam. Then I set the coated board directly on the pin points to dry (i.e. flat which tends to give it the best uniform coating). Others use a small wire attached usually via a hole and hang the board from it, but I found that causes to much of the polyurethane to drip off before it solidifies reducing the total amount of coverage. Although sunlight will yellow the polyurethane over time, I've had boards that look great even after years of being outside with no visible signs of corrosion. And if you have attached wires that can't be completely coated with the polyurethane, just put a few dabs of hot glue on them and usually your good to go on that as well. What I also like about this method is that if/when you need to do maintenance or modifications, aside from the nasty burning of the polyurethane (which you should be using an exhaust fan anyway), you generally can fix the problem and then just recoat the board with polyurethane again. With many (not all) conformal coatings this isn't possible to do (especially if you pot the board). One small can (1/2 pint) of polyurethane will generally run you around $7 at WalMart and will last for 100's of small sized boards (now that's cheap!). Of course aside from this, an enclosed box is always helpful to prevent the sunshine issue of oxidizing the polyurethane along with additional protection from really harsh weather conditions (i.e. like high winds causing debris to nick your board).