Potting for under water

[Robert Scott]

I have a circuit board that needs to be permanantely under water.
Does anyone know if potting with Polyester Resin Casting Compound
(sold in arts and crafts stores) will hold the water out indefinitely?

Same question for ordinary 2-part clear epoxy.


-Robert Scott
Ypsilanti, Michigan

 

[Roger Hamlett]

I have a circuit board that needs to be permanantely under water.
Does anyone know if potting with Polyester Resin Casting Compound
(sold in arts and crafts stores) will hold the water out indefinitely?

Same question for ordinary 2-part clear epoxy.

Get electronic potting compound instead.
There is normally a choice of epoxy, polyeurethane, and silicone
elastomer. The problem with normal resins, is that unless you investigate
the chemistry carefully, they may contain acids or alkalis, which can
themselves cause corrosion (Silicone RTV's, often for example use acetic
acid). Even though you use a potting compound, add a grommet around any
wire entrances (the tendency for water to penetrate 'along' a surface at a
joint is amazing, and you also want strain relief where the wire enters a
rigid material).

Best Wishes

 

[Mike Harrison]

I have a circuit board that needs to be permanantely under water.
Does anyone know if potting with Polyester Resin Casting Compound
(sold in arts and crafts stores) will hold the water out indefinitely?

Same question for ordinary 2-part clear epoxy.


-Robert Scott
Ypsilanti, Michigan

You should look in the catalogue of a specialist potting compound manufacturer (robnor, Dow Corning)
as there are a wide variety of types for different applications.

 

[Peter]

I have a circuit board that needs to be permanantely under water.
Does anyone know if potting with Polyester Resin Casting Compound
(sold in arts and crafts stores) will hold the water out indefinitely?

Same question for ordinary 2-part clear epoxy.


-Robert Scott
Ypsilanti, Michigan

There is not much of a problem in potting things so they are
immersible.

The fun starts when you want to have wires going into it.

Water will get in in between the strands of a stranded cable.

It will also get in between the encapsulant and the wire insulation -
unless you ensure there is a chemical bond. Usually this bond doesn't
occur; it's very hard to make anything stick to soft bendy plastic.
Many years ago I achieved this in production by using irradiated vinyl
insulation (high temperature rated) which adheres OK to epoxy resins.

You also have to watch shrinkage during curing - this can crush many
components. Some epoxy resins are particularly bad, especially the
thermally conductive ones.


Peter.

 

[chris]

Get electronic potting compound instead.
There is normally a choice of epoxy, polyeurethane, and silicone
elastomer. The problem with normal resins, is that unless you investigate
the chemistry carefully, they may contain acids or alkalis, which can
themselves cause corrosion (Silicone RTV's, often for example use acetic
acid). Even though you use a potting compound, add a grommet around any
wire entrances (the tendency for water to penetrate 'along' a surface at a
joint is amazing, and you also want strain relief where the wire enters a
rigid material).

Best Wishes

Does it *have* to be completely potted?

What about a thick layer of conformal coating? The ingress along wire
entrances issue still applies, however.

Chris.

 

[Joerg]

Hello Robert,

One of the industries you could check out (publications etc.) would be
undersea robots. These guys really know since their stuff is exposed not
just to water, but usually to salt water under extreme pressures.

Anyway, you also have to consider what kind of water your board will
have to survive in. Salt? Chlorine? ph value? Is it chemically stable?
How many years? Then there are some living underwater beasts that can
eat into almost anything.

We have a pool and over the years I have learned some hard lessons. even
the low doses of chlorine in a pool will eat away at stuff. Then, after
installing a water filter, big surprise: The tap water around here when
measured before the filter had the same chlorine level as our pool.

Regards, Joerg

 

[Nico Coesel]

Hello Robert,

One of the industries you could check out (publications etc.) would be
undersea robots. These guys really know since their stuff is exposed not
just to water, but usually to salt water under extreme pressures.

Anyway, you also have to consider what kind of water your board will
have to survive in. Salt? Chlorine? ph value? Is it chemically stable?
How many years? Then there are some living underwater beasts that can
eat into almost anything.

I have no experience at all in this field, but a gut feeling says:
take an enclosure, fill it with a liquid which doesn't harm the
circuit and make sure the inside pressure can be the same as the
outside pressure. Any comments?

 

[Joerg]

Hello Nico,

I have no experience at all in this field, but a gut feeling says:
take an enclosure, fill it with a liquid which doesn't harm the
circuit and make sure the inside pressure can be the same as the
outside pressure. Any comments?

Then the problem is reversed: How do you keep that pressure up over
months or years? Even a sealed container leaks some, unless the liquid
hardens in which case it basically becomes a potting compound.

Regards, Joerg

 

[qrk]

I have a circuit board that needs to be permanantely under water.
Does anyone know if potting with Polyester Resin Casting Compound
(sold in arts and crafts stores) will hold the water out indefinitely?

Same question for ordinary 2-part clear epoxy.


-Robert Scott
Ypsilanti, Michigan

Number of ways of doing this:
Potting
=======
If your depth isn't too great (couple 100 meters), you shouldn't have
to worry about parts crushing. Over 500 meters, you need to use parts
with no air cavity. Crystals and hermetically sealed packages have an
air cavity. Aluminum electrolytics don't handle pressure. To reduce
the chance of wicking, use a large solid bare wire with an o-ring over
the wire.

1. Polyurethane is commonly used in the undersea world. PR-1570 is
commonly used by the industry.
http://www.ppg.com/prc-desoto/pdf/pr1570_1_04.pdf

2. Various industrial expoies can be used.

3. Silastic J-RTV (Dow Corning). Has quite a bit of shrinkage.

Housings
========
4. Housing filled with inert fluid like castor oil or fluorinert.
Parts are subjected to water pressure. Useful mainly for very deep
housings where weight is a concern.

5. Standard pressure housing. Gets expensive with connectors and
tight-tolerance machining. See Parker o-ring manual for guidelines on
how to make the sealing areas. Use the commercial practices, not the
military practices in the Parker manual.

Mark

 

[Nico Coesel]

Hello Nico,


Then the problem is reversed: How do you keep that pressure up over
months or years? Even a sealed container leaks some, unless the liquid
hardens in which case it basically becomes a potting compound.

You don't need to keep the pressure up. A flat box will provide enough
flexibility to level the pressure between inside and outside. Or a
flexible membrane may be used. In my idea, equalizing pressures will
prevent liquid from leaking in or out.

 

[Glen Walpert]

Hello Nico,


Then the problem is reversed: How do you keep that pressure up over
months or years? Even a sealed container leaks some, unless the liquid
hardens in which case it basically becomes a potting compound.

Regards, Joerg

The problem is not leakage, which can be made negligible fairly
easily, but differential thermal expansion rates, which can not be
made negligible. A solution which has been used in liquid silicone
filled instruments is the metal bellows, which can allow for expansion
and contraction while maintaining internal pressure very close to
external pressure.

Regards, Glen