How to isolate a circuit from water

Hi all,
I'm developing a small PCB hosting:

  • an ATMega 328
  • a miny Lipo Battery 3.7 volt
  • a Lipo charger
  • 3 addressable LEDS

The pcb should be sporadically dipped in water, for about 1 second.
Questions:

  • Is there some water resistant resin I could use to cover and isolate the entire PCB?
    I think the PCB does not produce significant heat, so I guess this will be not a problem.
  • For Lipo recharge, I'm using a USB cable connected to a Lipo charger on the PCB. One end of the cable (that one I use for recharge) will not touch water but I need to isolate the other end (that one connecting on the Lipo charger).

Thanks for advices.

Hi,

google conformal coating

Tom... :slight_smile:

Even with conformal coating, you are likely to get corrosion set in. Water and electronics do not mix.
Sunlight + water will be even worse.

You need to pot the circuit in a box really, but a LiPo battery should never be potted, it becomes a
hidden fire risk.

Why do you want to do this?

Taking about conformal coating, is this conformal coating?
I have a can of that, and I cannot say I trust it a lot. Ok, it's a thin transparent film, so it is probably normal that I do not see a lot, but I also didn't have a lot of problems using a multimeter afterwards on the contacts. If it works at all, it does not give a lot of mechanical resistance.

I still use it a lot when 240V are involved, just as an additional layer of security. I do not reduce trace clearance because of that, though.

valerio_sperati:
The pcb should be sporadically dipped in water, for about 1 second.
Questions:

  • Is there some water resistant resin....

When you dip something in water you should look for water-proof resin and not water-resistant.

how deep in water? purity of water? is it saline? is it acidic? is it hard water?

valerio_sperati:
I think the PCB does not produce significant heat, so I guess this will be not a problem.

Your PCB does not produce much heat when it's open and ventilated. When it's enclosed it might start to build up heat. You should double check this first.

Hi,
The can you linked to is good for corrosion protection, but not for overall waterproofing.

What is your project that needs to be dipped in water?
What is the application?

Tom... :slight_smile:

Further considerations. Your PC board probably has wires attached. Wires are usually flexible. Water will follow the surface of the wires into the conformal coating because wire movement has made the coating release form the wire insulation. If water gets inside the wire insulation it will move eventually the circuit board.

Design your project so wires never get wet.

Paul

TomGeorge:
The can you linked to is good for corrosion protection, but not for overall waterproofing.

I am not the OP, I did not intend to use this to make anything waterproof. However, this stuff is called conformal coating in the datasheet, and I wanted to check if that is what you meant. Because, as I said, I really wouldn't trust it for that, and actually not really for anything regarding safety ..

What conformal coating will protect against is condensing humidity. It is good at that.

Grumpy_Mike:
What conformal coating will protect against is condensing humidity. It is good at that.

At least the one I linked seems to be super easy to mechanically damage, though. As I said, I had no problems holding a not very sharp DMM probe to a sprayed contact and get a reasonable reading.

Yes that is the point of conformal coatings. It is called probe through. It is meant for servicing. Once serviced you need a quick spray to it again, unless the coating is self healing.

The coating is ment for a board that is mounted in a box, not a free board that is handled.

Grumpy_Mike:
Yes that is the point of conformal coatings. It is called probe through. It is meant for servicing. Once serviced you need a quick spray to it again, unless the coating is self healing.

The coating is ment for a board that is mounted in a box, not a free board that is handled.

Ok, that is good to know. I was using it for stuff in boxes, mainly when mains is involved, but I was never quite sure if it makes any sense at all.

Hi all, thank for your advices. I attach here a sketch of my project, so to better illustrate my needs:

This is a device to be used by a bubble artist, on his hand; I write here a list of additional info:

  • he will dip his hand in water with soap, approximately up to the wrist; dipping time: about a couple of seconds. This is needed by the artist to soak the hand with water soap.
  • he will switch the leds on/off through two mini pushbuttons.
  • I’m still thinking how to arrange all stuff: probably I’ll sew everything on a glove worn by the artist.
  • the artist will wear a fabric wristband on the wrist, in order to avoid water dripping on the arm. Probably I’ll fix the pcb on this wristband: note that the wristband, even if not dipped directly into water, will be very very wet
  • The critical parts, as shown in the sketch, are the led strip, the pushbuttons and the wires connecting to the pcb: all these elements dip in water.

Now, according to you, how can I isolate all the circuit?
Do I need necessarily a waterproof box for the pcb?
How can I isolate leds, pushbuttons, and wires, which are necessarily out of the (possible) waterprof box?
Thanks a lot!

[/list]

As mentioned before, Google “potting compound”

Why not put the PCB further up his arm , so it doesn't get wet .

Even better!

hammy:
Why not put the PCB further up his arm , so it doesn't get wet .

Because when he raises his hands water will drip down his sleeves wetting the PCB.

So now we are on page 2 we finally get a full description of this former X-Y problem.

You can not use resin for this. It is a very difficult task, I would hesitate to take on a project like this. It will end in tears.

Try and remove the switches from the hand and have some other way of control like an under arm switch, or magnets and reed switches. The so called water proof LED strips are not water proof enough. To find out how water proof something is you need a measure known as an IP rating, led-ip-ratings-led-flex-strip

I think I would look at some fiber optics for the lighting

One important factor is the expected life or reliability. Are you trying to make a product or just a hobby project?

If a hobby project, you need to seal it well enough it will last a reasonable amount of time and it doesn't fail shortly after starting to use it.

Not sure what switch you have in mind so I'll put that aside for now. The LED can be purchased as "waterproof" in eBay. I would expect these to be at most water resistance but will likely work for some time.

For the PCB. One of the issues with conformal coatings is they don't cover sharp edges very well. And can be this in areas allowing moisture to eventually cheep in.

I would suggest a piece of heatshrink and an electronics grade silicone RTV. For hobby use I know you can't use the type that emits acetic acid when curing. You might be able to use the newer grades that don't smell as bad like GE Silicone II. I'm not sure what their curing byproducts are.

You should use an electronics grade (i.e. non corroding) of RTV. You can find them on Amazon, Digikey and likely other outlets.

For a trial you also might try hot melt glue. If you use the silicone wire I mention below you will get much better adhesion from the potting to the wire if you use a silicone sealant (aka potting compound).

The procedure I've used:

  • Coat the board and wires with sealant, lots of sealant, you want not to have any air trapped on the board
  • slide the heatshring over the board and sealant
  • Shrink the heatshring
  • allow to cure

A suggestion on wiring. If you want to be able to replace the LED and/or switch after the PCB is potted, I would simply run the wires up the arm to a relatively dry area and make the connections there.

Another word on wiring. I've recently found some neat very flexible silicone wire. It may be useful for your project.

ebay silicone wire
There are likely other vendors, but I only have wire from this vendor and am very happy with it.

Again if you are trying to design a saleable product none of the above will be adequate if you expect any reasonable kind of life.

Good luck.

First of all, thank to everybody. You are giving me a lot of interesting feedbacks.

I summarise here some answers to your posts:

  • about removing the pushbuttons: the proposed "reed switches" seem a good idea, I did not know them, I'll try them.
  • about the product: this is right now an hobby project but... the artist should be quite sure it works during its performance, so I need to be reasonably sure it does not fail. (If it works for a couple of performances, we will think about professional engeneering of it)
  • about fiber optics: I need a very bright light, I don't think fiber optic is fit for this.

After reading all your advices, I'm quite sure I need to build a water-proof protection box for the PCB (and Lipo!).
About leds, switches, and wires, I think I can isolate them using the proposed Electronic Grade RTV Silicone, the heatshrink tube, and the silicone wires (for optimal adhesion).

Thanks for sharing knowledge: I learned a lot of new terms: IP rating, Electronic Grade RTV, and X-Y problem definition.
I hope I'll manage to build a working prototype, then I'll continue to post here the result for new advices and comments.

Valerio