Big question is repairing the unit.

For non-repairable, I use a urethane two part mix.  It is fairly rubbery,
and can be stiffened and the thermal transfer improved by adding alumina
powder.  Can be cut with a hot knife, but is general not easily
repairable--you can get it off, but do more damage in the process.

For repairable, but much more fragile, I use a two part silicone, water
clear, very high surface resistance, cuts easily with a razor blade, and
peels off of PCBs--which makes repair very easy.  Both cure in about 4
hours at 50 degC.

At the present time I use the GE (?) RTV series with great success in our
small data loggers.  Have pretty much abandoned the use of the urethanes in
favor of the silicones.
Kelly





At 09:46 AM 3/13/00 +1000, you wrote:
>Hi,
>
>I need to encase an assembled PIC device in a polymer block.  For example,
>place the tested PCB+components in a mould and add a polymer so it sets
>around the board and components to make a waterproof case.  I also need to
>encase a sensor which needs only be used up to 100 degrees Celsius.
>
>I am concerned that the polymer, whether it be epoxy, neutral cure silicone
>or some other compound may change in resistance with time (the device uses
>15 bits for each measurement, so small changes count).  This change could be
>due to water absorption or structural reorganisation.
>
>Please share your successes and failures.  One problem with epoxy is the
>exothermic polymerisation, however, the setting temperature should not
>exceed 100 degrees Celsius.
>
>
William K. Borsum, P.E. -- OEM Dataloggers and Instrumentation Systems
<borsum@dascor.com> & <www.dascor.com>San Diego, California, USA