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

Bob kb8tq kb8tq at n1k.org
Sun Apr 9 14:34:01 EDT 2017


If anybody gets into this sort of thing in the future — There are black / optical
blocking die coat materials out there. They are silicone based and quite stable. 
We used a *lot* of the stuff on watch modules after it was discovered that the
watch died when exposed to a heavy dose of sunlight (right through the LCD
and into the chip … poof!!)


> On Apr 9, 2017, at 1:10 PM, David <davidwhess at gmail.com> wrote:
> 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.
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