I had occasion to pot very high impedance circuit in 2 part black
potting compound, when apparently set it failed to function correctly,
after 2 days all was ok.
my conclusion was that the chemical reaction was generating conducting
ions and conduction ceased when the reaction finished.

Dave Platt wrote:

Epoxy, as described by others, tends to stress the components - one
notable job I had was the vco in a Yaesu FT480R - took AGES to chip
away with a fine wood chisel after using a blowtorch to open up the
brass enclosure.....one of those "personal challenge" jobs.

Several notes re epoxy for encapsulation:

- Epoxy formulations vary a lot in their hardness and rigidity,
viscosity when fluid, cure rate, and rapidity of heat release
during cure. Most of the big epoxy companies seem to make at least
one variety which is specifically intended for use when
encapsulating electronic components. These formulas seem to be
more flexible when cured, and some of them are being advertised as
providing shock and thermal-stress protection.

- Other commercial electronic-gear encapsulants seem to be based on
urethane, or on two-part silicone rubber formulas.

- I've had some success in doing small encapsulating projects by using
a mixture of a low-viscosity slow-cure epoxy (one of the TAP
Plastics formulas) and fine white sand, mixed up in a thick slurry.
This mix is a good deal less expensive than using straight epoxy
resin, it has much less shrinkage, and should have rather better
thermal conductivity. When it's poured into a mold, the sand tends
to settle to the bottom before the epoxy hardens, leaving the top
layer in the mold as nearly-pure resin (a nice smooth surface).

--
Dave Platt AE6EO
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