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Date: Wed, 18 Jul 2012 10:18:12 +0200
From: Gabriel Paubert <paubert AT iram DOT es>
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Subject: Re: [geda-user] Soldering problems
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On Tue, Jul 17, 2012 at 11:15:21PM -0700, Joe Knapp wrote:
> I would like to add that water soluble flux can cause issues also. It
> can be conductive. I have seen ~50K Ohms between pads on SMT devices due
> to organic water soluble not being properly cleaned. This is a real
> issue with small pitch LDOs or LDOs with power pads. This can affect the
> set point of the adjustable LDO and cause un-desirable voltages.

Thanks for all the tips. Actually I'm going to have soon to assemble
a board with an ADG5212 or ADG5213. I've chosen these switches because
of the very low leakage and charge injection. The signal level will
be very low, and close to ground while the power supplies will be
symmetrical (+/-9V to +/-15V). 

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:

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.

	Regards,
	Gabriel