[Halld-cal] 2500 volt capacitors

[Fernando J. Barbosa]

Hi Paul,

Paul Smith wrote:
> Hi Fernando,
>
> I've tried several of these capacitors, AVX #2225WC103KAT1A, .01 uF @ 2500 volts.  I solder wires to the metallization, then connect to a high voltage power supply.
>
> The ones off of a cut tape I got from DigiKey typically spark at ~1500 volts, then stop.  This is independent of humidity from 40% to 55% RH.  If you gradually raise the voltage, they will spark at higher voltages, but eventually will hold 3 kV without sparking.
>
> I tried one from a reel I got from another distributor; it holds 3 kV at 53% RH without any "conditioning".  If I breath on it, it will start sparking, but then stops.
>
> If you look at the metallization on the ends of these caps under a microscope, it isn't anything close to smooth.  The 2nd batch is better than the first, but still not great.
>
> I'm pretty sure that the sparks burn off the "points" of the metallization.
>   
Is the metalization you refer to SnPb? If it is, it will most likely get 
better once the board is re-flow soldered in the oven as sharp edges 
will decrease.
> It's hardly practical to "burn-in" each of nearly 10,000 capacitors.  They will work with just the 1B31 acrylic conformal coating (and no epoxy), but I think it needs to wick under the capacitor as well so that the capacitor is surrounded by the coating.  I've been using some coating in a syringe which I squirt along the sides.  Removing a capacitor verifies that the coating does indeed wick underneath.
>   
I am glad the 1B31 will do the job. You can remove the solder mask from 
under the components, or partly, to provide more clearance under the 
components and make it easier for the 1B31 to get under.

Another option, and I think you already mentioned this some time ago, is 
to use two smaller caps in series but this depends on available space.

I agree that "burn-in" of the caps is not practical but you might 
consider a procedure to have the boards very well cleaned after 
assembly, followed by a 30 minute oven dry cycle at 50-70C or in a 
purged nitrogen atmosphere for a week and immediately followed by 
application of the conformal coating.
> I don't see how we can rely on these capacitors without some sort of coating surrounding them on all 4 sides.
>   
I am fine with the 1B31 if the boards can operate reliably. The nominal 
conditions in the hall will be 70F and 50% RH.

Thanks and best regards,
Fernando



> Paul
>   
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