[time-nuts] Re: Potting compound advice needed

[Brooke Clarke]

Hi Jim:

Be careful with RTVs.  Some out gas acid that attacks metal, even gold plated metal.  Guess how I know that.

-- 
Have Fun,

Brooke Clarke
https://www.PRC68.com
axioms:
1. The extent to which you can fix or improve something will be limited by how well you understand how it works.
2. Everybody, with no exceptions, holds false beliefs.

-------- Original Message --------
> 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.
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