[time-nuts] Re: Potting compound advice needed

[Lux, Jim]

On 11/10/21 2:40 PM, Richard (Rick) Karlquist wrote:
> I am looking for help choosing a potting compound that
> has the following properties:
>
> 1.  Good for 5,000VAC @ 1 MHz
> 2.  Low RF losses.
> 3.  Low permittivity is preferred
> 4.  Low tempco of permittivity is a want.
> 5.  Something I can implement in my home shop
> without access to a vacuum pump etc. is a want. 

What about curing? Is temperature cure (put it in an oven) ok? or do you 
need room temp cure?


Silicones are usually pretty good, RF wise. But you need to check the 
filler and exact composition.

I found a two component silicone that has epsilon 2.5 used for RF 
potting, 15kV/mm breakdown.

https://vitrochem.com/wordpress/wp-content/uploads/2020/10/Two-Component-Condensation-Silicone.pdf

they say nothing about the dissipation.


Aha.  RTV12 from Momentive - clear - epsilon 3.0, tan d (at 1kHz) is 
0.001, 400 V/mil - This stuff is pretty common, but I can't find any 
higher frequency permittivity info, which is odd. Someone somewhere 
probably built something and measured it.


Diallyl Pthalate is what they use in connectors - it's a thermosetting 
resin with good electrical properties.

https://www.cosmicplastics.com/products/dap/

Picking the first one in the list 224 DAP - 360 V/mil, so for your 5kV, 
you'd need ~14 mils. (most plastics are in this range)

Epsilon is kind of high 3.5, tan D is 0.01?  Is that good enough for you 
dissipation wise?  There's lots of kinds with various fillers.

A common way to reduce epsilon and tan d is to mix in microspheres.


Some epoxies are also good.  Rogers not only makes laminates for 
circuitboards they also produce the epoxy from which they are made


We use tons of arathane and solithane at JPL (both are urethanes), but I 
don't know if we pot RF circuits in araldite. Huntsman makes the 
"ara???" materials

https://huntsman-pimcore.equisolve-dev.com/Documents/US_2019_High_Performance_Components_Selector_Guide.pdf

one thing is that we store this stuff at -80C, but I don't know if 
that's after mixing or if it's shipped that way (in dry ice).

masterbond.com  -> give them a call or email

EP110F80-1 is a 2 part epoxy with e=2.69 at 1MHz, so it's probably 
reasonably low loss.