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Date: Wed, 18 Jul 2012 21:12:17 +0200
From: Gabriel Paubert <paubert AT iram DOT es>
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Subject: Re: [geda-user] Soldering problems
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On Wed, Jul 18, 2012 at 08:32:55AM -0700, Frank Miles wrote:
> On Wed, 18 Jul 2012, Gabriel Paubert wrote:
> 
> [snip]
> 
> >I've been worried about leakage due to surface currents, this is an area
> >in which I have little experience (in microwave at 50Ω characteristic
> >impedance, leakage due to a few tens of kΩ would be irrelevant). In 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 100µm track
> >width and spacing. The result is basically:
> 
> In the "old days" nodes like this would joined on teflon standoffs.  Guard
> rings are very helpful, of course.  Whether this can be made to work or not
> - not sure.

Hopefully it can, otherwise data sheet's values cannot be achieved.

> 
> >Power pad|100µm space|100µm ground|100µm space|Signal pad|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 150GΩ, which
> >I don't even know how to measure. I was thinking of cleaning up all residue
> >carefully (acetone and/or IPA). Then the board will be handled with gloves
> >and installed inside a shielded enclosure (no dust can come in).
> >
> >But still, I'd like advice from people who do low level signal measurements.
> 
> 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.

Thanks. Yes I was planning at least 2 steps of rinse.

> 
> Can you do anything to control (minimize) humidity?

I can install the board inside the enclosure in a very dry environment,
blowing dry air or even nitrogen around it. Once the enclosure is closed, 
it should be airtight.

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

That's the part of the problem I'm still exploring. I have been thinking
about it but until now I was mostly concerned by the assemly issues, so 
it's a bit premature to discuss these issues. I should get the board
next week, but the enclosure will not arrive before October.

	Regards,
	Gabriel