Cheap potting alternative [Design]

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From: legg on 7 Mar 2010 09:19

On Sun, 07 Mar 2010 07:28:00 -0500, Boris Mohar
<borism_void_(a)sympatico.ca> wrote:

>On Sun, 07 Mar 2010 04:09:22 GMT, larrykent(a)raglands.com (Larry Kent) wrote:
>
>>Is it possible to use epoxy or polyester surfboard resin for potting
>>audio fequency PCB's?
>>
>>How about the crystal epoxy used for encapsulating insects, etc?
>>
>>Any other low cost alternatives, aside from flexible rubber/silicone?
>>
>>Many thanks,
>>
>>Larry Kent
>>
>
>Wax.

=fuel

RL

From: MooseFET on 7 Mar 2010 10:21

On Mar 6, 8:09 pm, larryk...(a)raglands.com (Larry Kent) wrote:
> Is it possible to use epoxy or polyester surfboard resin for potting
> audio fequency PCB's?
>
> How about the crystal epoxy used for encapsulating insects, etc?
>
> Any other low cost alternatives, aside from flexible rubber/silicone?
>
> Many thanks,
>
> Larry Kent

How about, don't pot it. It is a low cost alternative you may not
have
considered.

I have seen boards potted in stuff that was basically tar. At about
100C
the stuff would melt and become a sort of sticky goo.

From: J.A. Legris on 7 Mar 2010 11:12

On Mar 6, 11:09 pm, larryk...(a)raglands.com (Larry Kent) wrote:
> Is it possible to use epoxy or polyester surfboard resin for potting
> audio fequency PCB's?
>
> How about the crystal epoxy used for encapsulating insects, etc?
>
> Any other low cost alternatives, aside from flexible rubber/silicone?
>
> Many thanks,
>
> Larry Kent

Cheapest alternative: don't pot - it's messy, insecure,
environmentally unfriendly, and makes service and repair just about
impossible.

Unless you intend to make it part of the surfboard, in which case you
should consider a sealed enclosure.

--
Joe

From: Dr. Heywood R. Floyd on 7 Mar 2010 11:41

On Sat, 6 Mar 2010 22:17:50 -0800, dplatt(a)radagast.org (Dave Platt)
wrote:

>>Is it possible to use epoxy or polyester surfboard resin for potting
>>audio fequency PCB's?
>>
>>How about the crystal epoxy used for encapsulating insects, etc?
>
>Polyester resins often heat up a lot, and shrink, when curing. That's
>not necessarily a problem when fiberglassing a surfboard, but it can
>put a serious stress on components during encapsulation - might crack
>'em loose from the board in a really severe case. I believe this
>tendency can be reduced somewhat by using a slower curing cycle (less
>catalyst per amount of resin).

One can also reduce this issue by conformally coating the PCB first.
That way, no potting compound migrates between SMT parts and the PCB ,
which is where these shear forces occur.
>
>Some of these resins remain somewhat sticky when cured, and need a
>topcoating or upper layer containing a surface-hardening agent or wax.
>
>You can "stretch" epoxy used for potting, and save quite a bit of
>money, by loading it up with a filler before pouring.

Fine grain fiberglass "powder" (shards) makes an excellent filler as it
also makes the media a better thermal conductor.

> I've
>encapsulated small circuit boards in TAP Plastics' slow-cure marine
>epoxy, mixed with fine dry sand... cheap and effective, if rather
>heavy and prone to settle out during the curing process. Glass
>microspheres are much lighter, don't form as hard a matrix, and are
>rather more expensive :-(

One can go to nearly any automotive machine shop, and can usually buy a
handful (or a quart sized volume) of glass beads used for grit blasting
purposes. You need to buy it from a newly opened bag that has not been
added to the grit blaster yet.

From: Archimedes' Lever on 7 Mar 2010 11:43

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