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

Tom Curlee tcurlee at sbcglobal.net
Sat Apr 8 13:49:01 EDT 2017

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?

      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.
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