Sealing LEDs for underwater marine use

[cpf user]

Having great difficulty trying to seal 3w stars for saltwater use. They are being attached to thin aluminum plates which function as heat sinks when out of the water, but in the water, the temp is really not a concern.

The problem has been sealing the LED-to-aluminum structure from electrical corrosion. So far have tried caulking silicone, LED-seal conformal coating, and Sylgard 184 solar-cell encapsulant. A few days is the longest it's lasted before shorting out. Even after cleaning the stars in alcohol and giving them 3 coats of the Sylgard, they shorted out in one day, right through the spot where the LED-leads attach to the pcb. When peeling off the silicone, I could find no place where water could be seeping into it.

The coating needs to be relatively thin so as to let the LEDs cool in the water, and of course it needs to be optically clear. It also need to be non-toxic to humans.

What am I doing wrong?

 

[Steve K]

I think you might benefit from an introduction to products known as potting or encapsulants. These are the materials used to protect electronics from moisture, impact, dirt, etc. Henkel makes a variety of encapsulants for electronics:
http://www.henkel.com/electronics-products-finder-26750.htm

Emerson & Cuming is another big manufacturer of this sort of product:
http://www.ellsworth.com/emerson-cuming/

My work experience with this sort of stuff is that it's difficult to do well. Preparing the surfaces of the material that the encapsulant will be bonding to is a science, and I mean that literally. If you don't understand what "surface energy" means, then you will be relying on good luck more than knowledge (I relied on people who did know what surface energy means!).

A method to find the path of the water leak is to get a bucket of water, add a UV dye, and dunk your device in the water. Let it soak, pull it out, wash off the outside, and let it dry. With a transparent coating, the leak can be found with a UV lamp.