[time-nuts] TAPR TICC boxed (input protection)

[David]

I have run across the conductive carbon filled plastic problem before.

We did not actually use just paint.  We took black mastic electrically
insulating tape, dissolved it in thinner, and painted the parts with
it.  It dried to form a pliable black coating.

On Sat, 8 Apr 2017 17:49:01 +0000 (UTC), you wrote:

>You need to be careful how you paint the package black.  My first electronics job was in a place that made, among other things, mass spectrometers.  We made very high input impedance electrometers for the mass specs using TO-5 can mosfet transistors.  One batch was found to be very photo sensitive through the glass/ceramic lead interface.  Someone had the idea to spray paint the bottom of the package with black paint.  Not a good idea. The black paint, likely loaded with carbon, decreased the electrometer input impedance by many orders of magnitude.  Considering that our electrometers had an input impedance of 1E-12 to 10E-15, even a fingerprint made a huge difference.  The carbon filled black paint was practically a short.
>Maybe an overcoat with silicone or some other type of low leakage sealant, then the black paint?
>
>Tom
>
>> From: David <davidwhess at gmail.com>
>> To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
>> Sent: Saturday, April 8, 2017 10:00 AM
>> Subject: Re: [time-nuts] TAPR TICC boxed (input protection)
>>   
>>On Fri, 7 Apr 2017 01:06:17 -0400, you wrote:
>>
>>....controlling the offset voltage.
>>
>>We ended up painting the diodes black after soldering.
>>
>>I have also heard of it happening with metal TO-18 packages through
>>the lead interface under the package.