Just doing a little thinking here and I was wondering what my options
are to protect circuitry from the elements. I was thinking of encassing
the whole controller (PIC, crystal, regulator, adc, etc..) in epoxy to
protect it from rain, snow, etc... I'm just curious if this would cause
the components to overheat and eventually breakdown. Please, if you have
experience with this sorta thing let me know what you've tried and whats
been succesfull.
Thank you
Charles
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Is the PCB totally open to the elements, or is it just that the
enclosure it is in isn't totally waterproof?
For the waterproof handheld PCs I design in my present job, it's the
enclosure that is totally sealed.
Most military electronics I worked on in my last job had the fully
populated PCBs sealed with a environmental barrier sealant called
'humiseal', and the enclosures (which aren't portable) weren't sealed
totally along the underside, so that any moisture ingress dripped
straight out the bottom.
As for epoxying, it should work so long as the epoxy is UV resistant,
and waterproof, and adheres fully to any cables coming through to the
outside world. Some potting epoxies give you better heatsinking
properties than unpotted boards.
For my home projects, thou, I just stick it in a waterproof box with
sealed connectors, or connectors shrouded on the underside if fixed in
position. That way I can alter / repair the unit. (without having to
dig out the epoxy first!)
______________________________ Reply Separator _________________________________
Subject: Encasing Pic in Epoxy
Author: PC:spam_OUTcjoachimTakeThisOuTHOTMAIL.COM at INTERNET-HUSKY
Date: 30/01/98 16:42
Hi Guys
Just doing a little thinking here and I was wondering what my options
are to protect circuitry from the elements. I was thinking of encassing
the whole controller (PIC, crystal, regulator, adc, etc..) in epoxy to
protect it from rain, snow, etc... I'm just curious if this would cause
the components to overheat and eventually breakdown. Please, if you have
experience with this sorta thing let me know what you've tried and whats
been succesfull.
Thank you
Charles
______________________________________________________
Get Your Private, Free Email at http://www.hotmail.com
There are two factors to consider:
Water vapor transfer and absorption.
Heat transfer.
Water protection requires high-density encapsulants like organic waxes
or gelled oils. Lower density plastics or epoxies will allow both
moisture transfer and absorption with time. Unfortunately the plastic
products do not have good thermal conductivity. Loaded epoxies can have
higher thermal conductivity.
Inorganic encapsulants like silicone have fair moisture characteristic
and good thermal conductivity.
If you use loaded silicone encapsulants the heat transfer will be even
better than with air.
Thickness helps delay the moisture transfer but also delays heat
transfer.
> ----------
> From: Charles Laforge[SMTP:.....cjoachimKILLspam@spam@HOTMAIL.COM]
> Reply To: pic microcontroller discussion list
> Sent: Friday, January 30, 1998 8:27 AM
> To: PICLISTKILLspamMITVMA.MIT.EDU
> Subject: Encasing Pic in Epoxy
>
> Hi Guys
>
> Just doing a little thinking here and I was wondering what my options
> are to protect circuitry from the elements. I was thinking of
> encassing
> the whole controller (PIC, crystal, regulator, adc, etc..) in epoxy to
> protect it from rain, snow, etc... I'm just curious if this would
> cause
> the components to overheat and eventually breakdown. Please, if you
> have
> experience with this sorta thing let me know what you've tried and
> whats
> been succesfull.
>
> Thank you
>
> Charles
>
> ______________________________________________________
> Get Your Private, Free Email at http://www.hotmail.com
>
>Charles wrote:
>Just doing a little thinking here and I was wondering what my options
>are to protect circuitry from the elements. I was thinking of encassing
>the whole controller (PIC, crystal, regulator, adc, etc..) in epoxy to
>protect it from rain, snow, etc...
>
I have had some experience in potting small high voltage power supplies.
The use of hard epoxy is not recommended - it can break components by
differential thermal expansion. Flexible epoxies made for circuit potting
(such as from Conap and others) are OK. We used to test power supplies for
thermal stress cracking by letting them sit in boiling water and then
dropping them in ice water, repeating this several times.
Dow Corning 2-part RTV silicones (catalyst types) work well but are expensive.
One of the best things for potting is beeswax - it is used to pot several
commercial sonar depth sounders for boats. I believe it is even more
impervious to water than epoxy and can be melted out for circuit repair.
However, the use of beeswax potting on line-powered circuits presents a
serious fire hazzard.
In general component overheating is not a problem for low-powered circuits
since most potting materials have thermal conductivities greater than air.
The exceptions are components using a heatsink and relying on air
convection for cooling or components running at a high temperature, such as
power resistors.
This is obviously a subject where the collective wisdom has plenty of
experience, so I won't reiterate.
One 'gotcha' that I haven't seen mentioned is ensuring that your
encapsulant will actually bond to the surfaces you are potting. For
example, PCBs with flux remaining and plastic mouldings (including looms
with moulded plugs) that still have release agent on them.
If there is a minute gap between the potting compound and the surface, it
will draw water in by capillary action.
For the same reason, don't use the potting as strain relief.
Steve.
======================================================
Very funny Scotty. Now beam down my clothes.
======================================================
Steve Baldwin Electronic Product Design
TLA Microsystems Ltd Microcontroller Specialists
PO Box 15-680 email: .....stevebKILLspam.....kcbbs.gen.nz
New Lynn, Auckland ph +64 9 820-2221
New Zealand fax +64 9 820-1929
======================================================
At 08:27 1998-01-30 PST, you wrote:
>Hi Guys
>
>Just doing a little thinking here and I was wondering what my options
>are to protect circuitry from the elements. I was thinking of encassing
>the whole controller (PIC, crystal, regulator, adc, etc..) in epoxy to
>protect it from rain, snow, etc... I'm just curious if this would cause
>the components to overheat and eventually breakdown. Please, if you have
>experience with this sorta thing let me know what you've tried and whats
>been succesfull.
>
>Thank you
>
>Charles
I am casting a little regulator in epoxy. 3k units so far.
The manufacturer can tell you the heat transfer properties.
I use a epoxi with 70% quartz powder (not sure of the english name is correct)
for many reasons: Better heat transfer, lower price per volume, more
environmental (less plastic), and a heat elongation more in correlation
with the glass reinforced PCB.
There are lots of brands and types of epoxy.
Thera are also lot of other plastics to cast in.
Another idea is to cast in silicone rubber. Thermal transfer vary. This is
highly flexible and will not cause mechanic stress on components caused by
thermal etc.
For outdoor or marine (I think) use the best is som e kins of very flexible
sticky silica gel. It will heal itself if you by example stick a needle in
it. It will also guarenteed stick to whatever is coming out or into your
PCB (cables, connecting rods...) so no water will find its way in. Other
material above may by time and wear crack in theese cases and let water in
along cables. You cant use It alone; you have to cast it in a pot or
something.
The silicone and silica gel is removable for testing and repair/upgrade
purposes. Both also come in transparent versions.
At 08:27 AM 01/30/98 PST, you wrote:
>Hi Guys
>
>Just doing a little thinking here and I was wondering what my options
>are to protect circuitry from the elements. I was thinking of encassing
>the whole controller (PIC, crystal, regulator, adc, etc..) in epoxy to
>protect it from rain, snow, etc... I'm just curious if this would cause
>the components to overheat and eventually breakdown. Please, if you have
>experience with this sorta thing let me know what you've tried and whats
>been succesfull.
>
>Thank you
>
>Charles
Be carefull if you're using SMT components. We've had some cases where
differential thermal expansion caused the epoxy to rip one or more SMT
components off. 'Course we run from -40 to 150 C; your milege may vary.
We have one design which has high internal dissipation: we add silica
(sand) to the epoxy to increase the heat transfer rate.
Mark Walter
What you point out is true - but even worse if the minute gap is sealed
to form a closed void on the surfaces, the void will collect the
moisture with time (from diffusion transmission). I was responsible for
medical implantable electronic designs while doing research at Stanford
University. I used a vaccum to take out trapped voids before the
encapsulant set. The surfaces were slightly etched with acetone and
tetrachlorofluorcarbon to give better adhesion. Narrow-molecular-weight
wax was the encapsulant with a thin silicone overcoat for body rejection
reasons. The water transmission, absorption and adsorption were all near
zero for years of direct exposure to a 3% NaCl solution. We were
building micropower high-impedance circuits and IC, sensitive to very
small amounts of moisture.
> ----------
> From: Steve Baldwin[SMTP:EraseMEstevebspam_OUTTakeThisOuTKCBBS.GEN.NZ]
> Reply To: pic microcontroller discussion list
> Sent: Saturday, January 31, 1998 3:45 AM
> To: PICLISTspam_OUTMITVMA.MIT.EDU
> Subject: Re: Encasing Pic in Epoxy
>
> This is obviously a subject where the collective wisdom has plenty of
> experience, so I won't reiterate.
> One 'gotcha' that I haven't seen mentioned is ensuring that your
> encapsulant will actually bond to the surfaces you are potting. For
> example, PCBs with flux remaining and plastic mouldings (including
> looms
> with moulded plugs) that still have release agent on them.
> If there is a minute gap between the potting compound and the surface,
> it
> will draw water in by capillary action.
> For the same reason, don't use the potting as strain relief.
>
> Steve.
>
> ======================================================
> Very funny Scotty. Now beam down my clothes.
> ======================================================
> Steve Baldwin Electronic Product Design
> TLA Microsystems Ltd Microcontroller Specialists
> PO Box 15-680 email: @spam@stevebKILLspamkcbbs.gen.nz
> New Lynn, Auckland ph +64 9 820-2221
> New Zealand fax +64 9 820-1929
> ======================================================
>
> From: Charles Laforge <KILLspamcjoachimKILLspamHOTMAIL.COM>
> To: RemoveMEPICLISTTakeThisOuTMITVMA.MIT.EDU
> Subject: Encasing Pic in Epoxy
> Date: Friday, January 30, 1998 11:27 PM
>
> Hi Guys
>
> Just doing a little thinking here and I was wondering what my options
> are to protect circuitry from the elements. I was thinking of encassing
> the whole controller (PIC, crystal, regulator, adc, etc..) in epoxy to
> protect it from rain, snow, etc... I'm just curious if this would cause
> the components to overheat and eventually breakdown. Please, if you have
> experience with this sorta thing let me know what you've tried and whats
> been succesfull.
>
> Thank you
>
> Charles
>
> ______________________________________________________
> Get Your Private, Free Email at http://www.hotmail.com
Subject: Re: Encasing Pic in Epoxy
--------------------------------------------------------------------------
-----
I used to manufacture sensors for military aircraft, the pcbs were potted
in clear silicon rubber compound, that was mixed with slate powder, which
gave it a greenish colour. these were fitted in stainless steel cases that
were then electron beam welded together. The slate gave very good thermal
conductivity,while the silicon rubber could flex slightly toprevent rip
off of the SMTs, it was also soft enough to remove if any repairs were
needed.
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