[volt-nuts] plastic caps on 3458A reference board

[Frank Stellmach]

Well,

my guess for poly-sulfone for the top cap was not that bad:

The HP LTZ circuit, and also the LM399, were heated to about 95°C, and 
therefore, you need a thermally insulating plastic, which can withstand 
that temperature.

Poly sulfone is able to do so, whereas poly-styrene is OK for 70°C 
maximum, like in 45°C designs.

It would be OK also for that amateur purpose, if it's thick enough: The 
inner layer would deteriorate over time, but the outer side would still 
isolate enough.

If the whole PCB is placed inside a shielding box, the solder side cap 
can be omitted, of course.
I would place a thin sheet of poly styrene underneath the PCB to 
thermally insulate the circuit.

Frank