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The question here is how much flux remains on boards under normal manufacturing conditions, how conductive is it, and does the conductivity change with humidity and temperature.

I know that for super sensitive electrometer amps the recommendations for the finished board include very extensive washing in multiple stages.

Dirk Bruere
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  • That depends on what you call normal. Don't just ask random people on the internet, read the datasheet for your flux and liaise with your normal assembly people. You're right that better manufacturing process is needed for better results. –  Nov 10 '17 at 13:29
  • @BrianDrummond I was hoping someone had some experience of the problem. I guess I should stick to Arduino problems :-) – Dirk Bruere Nov 10 '17 at 13:40
  • @BrianDrummond BTW, we have another engineer currently doing some experiments, and data requests out to our normal board assemblers concerning flux composition, properties and cleaning procedures. – Dirk Bruere Nov 10 '17 at 13:46
  • You're probably also aware of design techniques such as guard rings to minimise problems on critical nodes. Perhaps more detail - even hand assembly or wave/reflow - would help the question. –  Nov 10 '17 at 13:55
  • @BrianDrummond It's a surface mount board already in production where thermal stability is crucial. Fully automated production. Just eliminating all possible sources of error, starting with all resistors to 0.2 ppm/C etc. Relaying the board will come later, with the usual attention to thermal gradients etc. The board controls a microbalance with a dynamic measuring range of 10^6 – Dirk Bruere Nov 10 '17 at 13:58

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I have seen issues with no clean fluxes when doing very high impedance stuff (10G Ohm), judicious application of solvents and an ultrasonic cleaner cleared it up, IIRC the solvent was just IPA so no real need for anything particularly aggressive. Words were had with the assembly house and the problem never reappeared.

The ones to Really watch are the water washable fluxes, which are fine if they manage to get them all off, but tend to be bad news if left on the board, I had problems with this on a high voltage assembly.

Dan Mills
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