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'[EE] Potting boards...'
2009\07\02@031216 by Jesse Lackey

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(repost with [EE] tag.  sorry.)



Hello all, I have a client who wants to pot the board I'm making to make
it as indestructible as possible.

I haven't done this before, and wonder a few things.
Maybe someone would like to offer advice?

Heat.  The components being potted generate some heat, not a lot, and
there will be a heatsink of some sort on the underside of the board.
(only the top is being potted).  So I'm not too worried about this.

My main concern is what being potted may do the following component types:

Electrolytic caps: they have some sort of safety vent, right?  If
sealed, and should disaster occur somehow, I figure failing in potting
couldn't be any worse than failing in open air?

Inductors: I'm using 5 for dc/dcs, 4 of which handle significant current
(2amps) in pulses.  They get pretty warm.  I just wonder if the potting
compound could cause them to change behavior (inductance value, current
capacity) in a significant way, by possibly squeezing them a little when
it cures?  Seems unlikely, thought I'd ponder.

Crystal: there is a 10mhz xtal that needs to be reasonably accurate
(250K baud serial coming in), and I remember somebody here saying once
how potting a board made their xtal drift.  How problematic might this be?

Any advice appreciated.

Thanks all-
J




2009\07\02@042112 by Clint Sharp

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In message <spam_OUT4A4C5DCC.9090309TakeThisOuTspamcelestialaudio.com>, Jesse Lackey
<.....jsl-mlKILLspamspam@spam@celestialaudio.com> writes
>My main concern is what being potted may do the following component types:
>
>Electrolytic caps: they have some sort of safety vent, right?  If
>sealed, and should disaster occur somehow, I figure failing in potting
>couldn't be any worse than failing in open air?
>
 I've had a capacitor in a small (0.5w) epoxy potted DC-DC converter
explode in front of my face, fortunately I was wearing eye protection
but I wasn't wearing ear plugs, took about an hour for the ringing to
stop. I never found the chunk that was blown off the module but judging
by the shape of the remains, it must have been rather unpleasantly
jagged.

>Thanks all-
>J
>
>
>
>

--
Clint Sharp

2009\07\02@115327 by Dwayne Reid

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A significant amount of industrial electronics is potted.  No problems.

You should be concerned with internal heat build-up.  How hot do the
inductors get?

I've used several gallons of potting compound over the years and
don't recall any problems developing because of it.  Don't recall if
anything had crystals in it, though.

Your best bet is to obtain some of the potting compound that you plan
to use and see what happens to your product.

dwayne


At 01:12 AM 7/2/2009, Jesse Lackey wrote:

>Hello all, I have a client who wants to pot the board I'm making to make
>it as indestructible as possible.
>
>Heat.  The components being potted generate some heat, not a lot, and
>there will be a heatsink of some sort on the underside of the board.
>(only the top is being potted).  So I'm not too worried about this.


--
Dwayne Reid   <dwaynerspamKILLspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2009\07\02@124411 by Funny NYPD

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Greeting Dwayne,
Do you have a supplier or vendor to share. And what's the normal price on those material?

Funny N.
Au Group Electronics, http://www.AuElectronics.com
http://www.AuElectronics.com/products
http://augroups.blogspot.com/




________________________________
From: Dwayne Reid <.....dwaynerKILLspamspam.....planet.eon.net>
To: Microcontroller discussion list - Public. <EraseMEpiclistspam_OUTspamTakeThisOuTmit.edu>
Sent: Thursday, July 2, 2009 11:53:24 AM
Subject: Re: [EE] Potting boards...

A significant amount of industrial electronics is potted.  No problems.

You should be concerned with internal heat build-up.  How hot do the
inductors get?

I've used several gallons of potting compound over the years and
don't recall any problems developing because of it.  Don't recall if
anything had crystals in it, though.

Your best bet is to obtain some of the potting compound that you plan
to use and see what happens to your product.

dwayne


At 01:12 AM 7/2/2009, Jesse Lackey wrote:

>Hello all, I have a client who wants to pot the board I'm making to make
>it as indestructible as possible.
>
>Heat.  The components being potted generate some heat, not a lot, and
>there will be a heatsink of some sort on the underside of the board.
>(only the top is being potted).  So I'm not too worried about this.


--
Dwayne Reid   <dwaynerspamspam_OUTplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2009\07\02@124457 by Marcel Duchamp

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Jesse Lackey wrote:
> Hello all, I have a client who wants to pot the board I'm making to make
> it as indestructible as possible.
>
> I haven't done this before, and wonder a few things.
> Maybe someone would like to offer advice?

Some issues to think about...

1. Thermal conductivity: how readily will heat get out?

2. Mechanical stresses: if the potting compound has a significant
thermal expansion coefficient, stresses can tear parts off the board.
This can be either during curing of the compound or after it has cured
if it is subjected to heat/cold or both.

3. Dielectric of the compound: not an issue with low impedance parts
usually but something to test for.  Run tests before and after potting
to see if frequency points shift or hi-z circuits operate differently.

4. Repair: usually not possible.

These issues can only be resolved by you knowing your circuit
characteristics and running compatibility tests.  In the case of
mechanical stress, sensitive parts may be protected by first coating
lightly with something like RTV silicone and then coating with the final
hard coat compound.

2009\07\02@131243 by Tony Vandiver

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I've used Stycast 2850GT - It's expensive, and pretty messy, and you
need to be aware that it's going to get hot while curing so some not so
high temp parts on the board could tend to melt, but once it's done, the
thermal conductivity is a nice plus since the whole thing essentially
becomes a heat sink.

Tony


Funny NYPD wrote:
> Greeting Dwayne,
> Do you have a supplier or vendor to share. And what's the normal price on those material

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