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'[EE] Silcone caulk as a conformal coat??'
2006\07\11@203255 by Timothy Weber

face picon face
I have a client who's happy with a gadget I made for him, but it's used
inside a walk-in cooler and gets dripping wet with condensation, and
therefore has problems due (we are assuming) to shorts.  I suggested
conformal coating.

It would be fastest and most convenient if he could just open up the box
and do it himself on this prototype - saves sending it back and forth to
me, etc. - but he'd also rather use some handy substance than wait to
order something special.  Specifically, he wants to cover the board with
silicone caulking.

This is... unusual... but it seems like it shouldn't hurt.  Any reasons
why it would be bad, other than that it would make repair practically
impossible?  Any alternatives that he could find at a local chain store
in upstate New York?  (Radio Shack apparently had nothing; I'm thinking
Home Depot or the like.)
--
Timothy J. Weber
http://timothyweber.org

2006\07\11@210412 by Martin Klingensmith

flavicon
face
Most silicone caulk has acetic acid in it. This will cause corrosion. It
has been mentioned recently something to the effect of: "it probably
doesn't matter, I've done it ..." and other comments like "don't use it"
or "coat the board with something else first"
This topic has been covered many many times in the past year I'd say,
try searching the archives if nobody else responds.
I would think, though I've never done it, that you could spray first
with polyurethane and then coat with epoxy or silicone. There are, of
course, special potting compounds $$

--
Martin K

Timothy Weber wrote:

{Quote hidden}

2006\07\11@211335 by Jinx

face picon face

> inside a walk-in cooler and gets dripping wet with condensation

Your first line of defence would be the case - can that be sealed ?
Inside you could give it a generous spray of polyurethane

2006\07\11@212325 by Marcel Birthelmer

picon face
What about dabs of super glue or hot glue on any exposed contacts (pins,
vias, etc.)? That may or may not be feasible depending on your type of board
etc.

On 7/11/06, Jinx <spam_OUTjoecolquittTakeThisOuTspamclear.net.nz> wrote:
>
>
> > inside a walk-in cooler and gets dripping wet with condensation
>
> Your first line of defence would be the case - can that be sealed ?
> Inside you could give it a generous spray of polyurethane
>
> -

2006\07\11@215003 by Timothy Weber

face picon face
Martin Klingensmith wrote:
> Most silicone caulk has acetic acid in it. This will cause corrosion.

Good to know - I was looking for a reason to do "the usual thing."

> I would think, though I've never done it, that you could spray first
> with polyurethane and then coat with epoxy or silicone. There are, of
> course, special potting compounds $$

Right, just wanted to avoid the delay of getting it to him - but if
there's a good reason we'll do it.

Jinx wrote:
>> inside a walk-in cooler and gets dripping wet with condensation
>
> Your first line of defence would be the case - can that be sealed ?

It can, but his experience with other gadgets in there has been, as
folks have said here recently, that unless you have NO vapor in the box
when you seal it, it'll condense inside the nicely sealed box anyway.
So he has had to drill holes in some other sealed boxes recently, to let
the water drain out.

I had thought about silica gel, but there'd be nothing to dry it out,
other than manually rotating packets, which is too much work.

Marcel Birthelmer wrote:
> What about dabs of super glue or hot glue on any exposed contacts (pins,
> vias, etc.)? That may or may not be feasible depending on your type
of board
> etc.

It's a prototype done quickly on perfboard, so the basic problem is
there isn't even any solder mask.  So this would amount to covering a
WHOLE lot of it with glue.  Seems yucky... but I'll keep it in mind.

Thanks all!
--
Timothy J. Weber
http://timothyweber.org

2006\07\11@215522 by Bob Axtell

face picon face
Timothy Weber wrote:
> I have a client who's happy with a gadget I made for him, but it's used
> inside a walk-in cooler and gets dripping wet with condensation, and
> therefore has problems due (we are assuming) to shorts.  I suggested
> conformal coating.
>
> It would be fastest and most convenient if he could just open up the box
> and do it himself on this prototype - saves sending it back and forth to
> me, etc. - but he'd also rather use some handy substance than wait to
> order something special.  Specifically, he wants to cover the board with
> silicone caulking.
>
> This is... unusual... but it seems like it shouldn't hurt.  Any reasons
> why it would be bad, other than that it would make repair practically
> impossible?  Any alternatives that he could find at a local chain store
> in upstate New York?  (Radio Shack apparently had nothing; I'm thinking
> Home Depot or the like.)
>  
What he is describing is also called bathtub caulk. There are two types:
vinegar-curing, and non-vinegar
curing. Unfortunately, the vinegar (acetic acid) attacks metal and might
not be good for electronics.
Actually silicone is an excellent choice for this application, but there
should first be a plastic spray applied to
the PCB directly, to protect the metal, then apply the other stuff.
Allow the plastic spray to dry 24hrs before
applying the silicone.  I use it to protect crystals from vibration
damage, by attaching the wires then  pushing
the crystal into a dollop of the silicone stuff stuck to the PCB. Kills
vibration damage to crystals.

A better candidate is some stuff made by "Rustoleum". Its called
"flexible rubber coating" and comes in a 12-oz
tall can. It is supposed to be used to coat pliers and other tools. It
also produces an awesome, strong, waterproof
high-dielectric strength coating. I normally dip the PCB once, then dip
it again 2 hrs later. The combination is thick
and very strong. I have used it to make motor control modules containing
PICs powered directly off of the AC line
and never had a problem. It cures with xylene, so you need an open
space- don't bottle it up in a garage or you might
get a noisy surprise. Full drying of 2 coats takes  24-36 hrs. You can
buy the stuff at ACE Hardware, in yellow, red,
black, blue, and white colors. Note: black is not recommended for
electronics (carbon dust).

None of these things should be applied _IN_ a cooler. They should be
applied and allowed to harden at room
temperature.

I have a JPG of the Rustoleum-coated PCB already dry, if anybody wants
it, email me offline.

--Bob

2006\07\11@224541 by Jinx

face picon face
>  > Your first line of defence would be the case - can that be sealed ?
>
> It can, but his experience with other gadgets in there has been, as
> folks have said here recently, that unless you have NO vapor in the box
> when you seal it, it'll condense inside the nicely sealed box anyway.
> So he has had to drill holes in some other sealed boxes recently, to let
> the water drain out.

You could put it in a warm oven or blow-dry it and then seal it ?

Can you add a heating resistor to the circuit ? When you say "cooler"
is that a chilled room or a frrrreeeeezzzer ? How much wattage do
you think might keep the inside of the box warm enough to stop the
condescension ?

2006\07\11@230905 by Martin Klingensmith

flavicon
face
Jinx wrote:

>> > Your first line of defence would be the case - can that be sealed ?
>>
>>It can, but his experience with other gadgets in there has been, as
>>folks have said here recently, that unless you have NO vapor in the box
>>when you seal it, it'll condense inside the nicely sealed box anyway.
>>So he has had to drill holes in some other sealed boxes recently, to let
>>the water drain out.
>>    
>>
>
>You could put it in a warm oven or blow-dry it and then seal it ?
>
>Can you add a heating resistor to the circuit ? When you say "cooler"
>is that a chilled room or a frrrreeeeezzzer ? How much wattage do
>you think might keep the inside of the box warm enough to stop the
>condescension ?
>
>  
>
That's actually a pretty good idea I think, Jinx.
I was thinking of this for a project I'm working on where it is exposed
to automotive temps, which could then adversely affect the components
that aren't rated for that low a temp. A power resistor positioned near
the sensitive components (in my situation) could raise the nearby area
to a more moderate temperature.

--
Martin K

2006\07\12@093132 by alan smith

picon face
Another thought....to make the client happy AND...since it appears that the proto is failing due to possible moisture issues....
 
 Just use the normal silicon...fill it up.  This does two things for you Tim.  First, it identifies the failure is indeed coming from the moisture in the system and second helps validate the design.  The amount of time it takes for the acidic properties to attack and destroy the circuit will be much longer than rebuilding a new unit, maybe on a regular PCB, conformal coat with the proper material and send him a new one.  Even if this is a one-off unit (is it?) it will keep your client happy.
 
 My worries would be power supply.  Unless you seal in the batteries (assuming its running on batteries) that would tend to be a problematic area reguardless.

Bob Axtell <.....engineerKILLspamspam@spam@cotse.net> wrote:
 Timothy Weber wrote:
{Quote hidden}

What he is describing is also called bathtub caulk. There are two types:
vinegar-curing, and non-vinegar
curing. Unfortunately, the vinegar (acetic acid) attacks metal and might
not be good for electronics.
Actually silicone is an excellent choice for this application, but there
should first be a plastic spray applied to
the PCB directly, to protect the metal, then apply the other stuff.
Allow the plastic spray to dry 24hrs before
applying the silicone. I use it to protect crystals from vibration
damage, by attaching the wires then pushing
the crystal into a dollop of the silicone stuff stuck to the PCB. Kills
vibration damage to crystals.

A better candidate is some stuff made by "Rustoleum". Its called
"flexible rubber coating" and comes in a 12-oz
tall can. It is supposed to be used to coat pliers and other tools. It
also produces an awesome, strong, waterproof
high-dielectric strength coating. I normally dip the PCB once, then dip
it again 2 hrs later. The combination is thick
and very strong. I have used it to make motor control modules containing
PICs powered directly off of the AC line
and never had a problem. It cures with xylene, so you need an open
space- don't bottle it up in a garage or you might
get a noisy surprise. Full drying of 2 coats takes 24-36 hrs. You can
buy the stuff at ACE Hardware, in yellow, red,
black, blue, and white colors. Note: black is not recommended for
electronics (carbon dust).

None of these things should be applied _IN_ a cooler. They should be
applied and allowed to harden at room
temperature.

I have a JPG of the Rustoleum-coated PCB already dry, if anybody wants
it, email me offline.

--Bob

2006\07\12@162744 by Timothy Weber

face picon face
First off - the client went ahead and used the silicone caulk before I
got back to him.  But, this is a prototype to prove the concept quickly
ahead of a more finished design, so we'll hope that by the time the
acetic acid digs in hard, he'll be using the Mark II.

Bob Axtell wrote:
> A better candidate is some stuff made by "Rustoleum". Its called
> "flexible rubber coating" and comes in a 12-oz
> tall can.

Darn, I *do* remember you saying this before and it's brilliant!  Would
have been a good alternative.  I'll keep it in mind for next time, or
perhaps the finished version - do you think it's cost-competitive with,
say, Techspray's acrylic coating or something similar?  I'm thinking
it's a pretty thick coat, if I'm envisioning the right stuff.

> Note: black is not recommended for
> electronics (carbon dust).

Great thought.

> None of these things should be applied _IN_ a cooler. They should be
> applied and allowed to harden at room
> temperature.

Yes, right - this isn't permanently attached or anything.

> I have a JPG of the Rustoleum-coated PCB already dry, if anybody wants
> it, email me offline.

Yeah, I'd like to take a look if you don't mind.

Jinx wrote:
> You could put it in a warm oven or blow-dry it and then seal it ?

Would be interesting to try... but seems risky without other protection,
and maybe not necessary if other protection is used?

> Can you add a heating resistor to the circuit ? When you say "cooler"
> is that a chilled room or a frrrreeeeezzzer ? How much wattage do
> you think might keep the inside of the box warm enough to stop the
> condescension ?

Ooh, very interesting idea.

It's a cooler - 35-38 F.  And actually, the gadget doesn't produce much
heat itself, but building a small heater into it might dovetail with
other aspects of the project nicely...  Hm!  I will ponder.

>   Just use the normal silicon...fill it up.  This does two things for you Tim.  First, it identifies the failure is indeed coming from the moisture in the system and second helps validate the design.  The amount of time it takes for the acidic properties to attack and destroy the circuit will be much longer than rebuilding a new unit, maybe on a regular PCB, conformal coat with the proper material and send him a new one.  Even if this is a one-off unit (is it?) it will keep your client happy.

My thoughts exactly.  If this one continues to work, the second version
will probably be done in at most a couple of months, maybe more like a
month.

>   My worries would be power supply.  Unless you seal in the batteries (assuming its running on batteries) that would tend to be a problematic area reguardless.

It uses a regulated wall wart supply that sits outside the cooler, so I
think it's still just a matter of ensuring the connections to the PCB
don't short.

Thanks again to all!
--
Timothy J. Weber
http://timothyweber.org

2006\07\15@070447 by Howard Winter

face
flavicon
picon face
Timothy,

On Tue, 11 Jul 2006 21:50:02 -0400, Timothy Weber wrote:

>...
> It's a prototype done quickly on perfboard, so the basic problem is
> there isn't even any solder mask.  So this would amount to covering a
> WHOLE lot of it with glue.  Seems yucky... but I'll keep it in mind.

If he just opens the box and squirts stuff in, only the front surface of the board will be properly covered.  
Murphy says the other side will have the condensation problem!  :-)

Cheers,


Howard Winter
St.Albans, England


2006\07\15@114333 by Timothy Weber

face picon face
Howard Winter wrote:
> If he just opens the box and squirts stuff in, only the front surface of the board will be properly covered.  
> Murphy says the other side will have the condensation problem!  :-)

Thanks!  I did mention that to him.  Bye bye, rework... well, it was
ugly anyway.

And - the globs of caulk completely solved the intermittent problems.
He's just going to watch it for a week or two before we move on to a
tweaked design and a larger run.  Yay!
--
Timothy J. Weber
http://timothyweber.org

2006\07\16@101514 by Robert Ammerman

picon face
Doesn't aquarium grade silicon caulk avoid the acetic acid problem?

Bob Ammerman
RAm Systems

----- Original Message -----
From: "Howard Winter" <HDRWspamKILLspamH2Org.demon.co.uk>
To: "Microcontroller discussion list - Public." <.....piclistKILLspamspam.....mit.edu>
Sent: Saturday, July 15, 2006 7:04 AM
Subject: Re: [EE] Silcone caulk as a conformal coat??


{Quote hidden}

> --

2006\07\16@223218 by Rich

picon face
I don't know if that silicone stuff is hygroscopic. I have never looked into
it.  My intuitive response if maybe it will have some undesirable effects.
If the stuff is hygroscopic, and your circuit cannot tolerate surface
leakage across the board, you might want to consider investing the
properties of the silicone based stuff.  Usually fet analog devices with
HI-Z in and small offset currents like very dry clean surface.

{Original Message removed}

2006\07\16@223420 by Rich

picon face
I should have read the entire thread.  I am glad that it worked.

----- Original Message -----
From: "Timothy Weber" <EraseMEtwspam_OUTspamTakeThisOuTtimothyweber.org>
To: "Microcontroller discussion list - Public." <piclistspamspam_OUTmit.edu>
Sent: Saturday, July 15, 2006 11:43 AM
Subject: Re: [EE] Silcone caulk as a conformal coat??


> Howard Winter wrote:
> > If he just opens the box and squirts stuff in, only the front surface of
the board will be properly covered.
{Quote hidden}

> -

2006\07\17@022202 by Russell McMahon

face
flavicon
face
> Doesn't aquarium grade silicon caulk avoid the acetic acid problem?


"Neutral cure" silicon rubber liberates (AFAIR) ethyl alcohol rather
than acetic acid during curing. This is less likely to do nasty things
to surrounding materials. Just don't lick too much of it until it's
properly dry.


       RM

2006\07\17@215955 by Bob Axtell

face picon face
Rich wrote:
> I don't know if that silicone stuff is hygroscopic. I have never looked into
> it.  My intuitive response if maybe it will have some undesirable effects.
> If the stuff is hygroscopic, and your circuit cannot tolerate surface
> leakage across the board, you might want to consider investing the
> properties of the silicone based stuff.  Usually fet analog devices with
> HI-Z in and small offset currents like very dry clean surface.
>  
Of course, it is always best to decide these things for yourself... but
here in the colonies, silicone caulk is
used to seal walls around bathtubs. The stuff absolutely stops water
from getting into the walls from the
mating of the bathtub and the wall. It is NOT hydroscopic (sp?).

--Bob

2006\07\18@102637 by Robert Young

picon face
{Quote hidden}

Be careful using RTVs that have acedic acid in them.  Most of the stuff
meant for use around the house has acedic acid (smells like cheap white
vinegar).  MG Chemicals and other companies make electronics grade RTV
silicone which might be more suitable.  Takes a bit longer to cure and
doesn't seem as "sticky" to me however.

Also, in general, RTV silicone has a very low slump factor so you might
need to mechanically assist its flow into the deep recesses of a box.

Rob

2006\07\18@131919 by Rich

picon face
Thank you, Bob:
Something for me to keep in mind for future reference.  I have sometimes run
into problems where a small surface leakage across the board generated gross
measurement errors because of the high input impedance devices.  I had never
considered silicone caulk.

----- Original Message -----
From: "Robert Young" <RemoveMErwyoungTakeThisOuTspamieee.org>
To: "'Microcontroller discussion list - Public.'" <spamBeGonepiclistspamBeGonespammit.edu>
Sent: Tuesday, July 18, 2006 10:26 AM
Subject: RE: [EE] Silcone caulk as a conformal coat??


>> {Original Message removed}

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