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Date: Wed, 18 Jul 2012 08:32:55 -0700 (PDT)
From: Frank Miles <fpm AT u DOT washington DOT edu>
To: geda-user AT delorie DOT com
Subject: Re: [geda-user] Soldering problems
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On Wed, 18 Jul 2012, Gabriel Paubert wrote:

[snip]

> I've been worried about leakage due to surface currents, this is an are=
a
> in which I have little experience (in microwave at 50=CE=A9 characteris=
tic
> impedance, leakage due to a few tens of k=CE=A9 would be irrelevant). I=
n this
> circuit, there are up to 15V of potential difference between a pin
> and its very sensitive neighbour, where a leakage of 1nA would swamp
> the signal. The spcaing between the pads is only 0.3mm! As a precaution=
,
> I've been inserting guard ring in which forces me to use 100=C2=B5m tra=
ck
> width and spacing. The result is basically:

In the "old days" nodes like this would joined on teflon standoffs.  Guar=
d
rings are very helpful, of course.  Whether this can be made to work or n=
ot
- not sure.

> Power pad|100=C2=B5m space|100=C2=B5m ground|100=C2=B5m space|Signal pa=
d|Space|Signal pad|...

>
> I believe I can not do much better in terms of layout, but what should =
I
> do during assembly to ensure that the leakage due to potential residues
> between signal and power pad is ~100pA or less. 15V/100pA is 150G=CE=A9=
, which
> I don't even know how to measure. I was thinking of cleaning up all res=
idue
> carefully (acetone and/or IPA). Then the board will be handled with glo=
ves
> and installed inside a shielded enclosure (no dust can come in).
>
> But still, I'd like advice from people who do low level signal measurem=
ents.

You're probably already planning on it, but I'd recommend a series of
rinse operations, each using clean solvents, and draining carefully
between steps.

Can you do anything to control (minimize) humidity?

How are you going to test whether you've been successful or not?

HTH!
 	-Frank
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