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Colleagues,

EEG signals typically have high source impedance. Amplifiers in the EEG circuits have high input impedance too. The signal is DC-coupled to the amplifier.

One problem is that leakages across the surface of the board can distort the signal. Guard rings help prevent that. Contamination on the PCB increase leakages. So, I would like to clean the contamination. Could anyone recommend a procedure for that and what solvents to use?

The board is FR4 with solder mask and silk screen. SMT components.

Cleaning will be done in an industrial facility, which is equipped for handling and disposing solvents. We have fume hoods.

UPDATE: Just added a bounty to this question. What I'd like to get is a fairly detailed procedure. Just throwing names of chemicals at this question will not count.

UPDATE: Bounty period had expired. Still looking for the procedure. Didn't get answers I could award the bounty to.
The tips which got posted so far might be useful for somebody in the future. More tips about board cleaning here (parallel thread on another forum).

UPDATE: Found more systematic insight into PCB cleaning (here and here).

Nick Alexeev
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    This question covers a lot of similar ground to http://electronics.stackexchange.com/questions/24428/which-chemicals-can-be-used-to-clean-electronics – Connor Wolf Mar 07 '12 at 07:18
  • Start with the more obvious like Isopropyl Alcohol. Eneding with something like Carbon Trtachloride is probably useful - noting that it's deemed carcinogenic and is a liver poison and can be absorbed into the body directly through the skin. Sniffing the fumes is not usually a good idea. Apart from all that it works very well. Use of a suitable conformal coating thereafter is a good idea. Use of eg [parylene](http://en.wikipedia.org/wiki/Parylene) as part of the manufacturing priocess means you tend not to have these problems as badly. – Russell McMahon Mar 07 '12 at 11:06
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    Out of curiosity, how much has cleaning the boards improved signal fidelity? – tyblu Mar 07 '12 at 14:38
  • @FakeName - they're definitely related, and it's good to have it in the "linked" category on the right, but I think that the context of cleaning to reduce surface leakage on high-impedance circuitry is unique enough for us to have multiple questions on the topic. – Kevin Vermeer Mar 07 '12 at 16:51
  • You're not going to get a detailed procedure without a LOT more information on your exact situation. – Connor Wolf Mar 15 '12 at 04:31
  • You say you're in an industrial facility that apparently does enough chemistry to have fume hoods. Have you considered asking the chemist who uses the fume hoods? – Connor Wolf Mar 15 '12 at 04:32
  • @FakeName I'd be glad to provide more information. Please do ask. Or, just post a relevant procedure. What if it's a good one? – Nick Alexeev Mar 15 '12 at 05:26
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    @FakeName The chemists at our facility, who use hoods are... just me. I can make an educated guess as to what the cleaning procedure should be. I can try to compile a procedure out of disjointed tips. But, I'd like to avoid reinventing the wheel by trial and error. This kind of cleaning have been done many times in different industries. – Nick Alexeev Mar 15 '12 at 05:27

3 Answers3

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This may be deemed a comment more than an answer, but just as important as which solvent (if any) to use -- we wash our boards with critical circuitry in RO (reverse-osmosis) water because it is free of salts + other contaminants -- make sure the boards are dried thoroughly.

Surface-mount chips often have a very thin gap between the chip and the board which can act as a capillary to retain water for days. So drying thoroughly is important. I'm not sure what works best, but in one case we put our circuit boards in a thermal chamber at a high temperature (probably 60-80 C; I forget) and low humidity for a day to drive off the water.

Jason S
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Very clean (deionized) water is actually a pretty good solvent for the nasty ions that allow conduction. You can get pretty good results by washing a board in a normal dish washer, then following up with a deionized water rinse.

However, this all misses the point. Even if you clean the boards really well and get the necessary high impedance between adjacent traces, then what? Without some way of making sure the board stays clean, the initial cleaning is rather pointless and actually gives a false sense of performance. Then there are things you can't clean afterwards. If the fiberglass isn't exactly right or isn't treated exactly right, ions can get trapped inside it that external cleaning isn't going to fix.

The better answer is to design the board to tolerate some amount of dirt. As you said, guard traces is one way. In rare cases you may need special glass standoffs and the like, but with good design you can usually mitigate the problem so that the board still works over a reasonable lifetime in the real world.

 

Olin Lathrop
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    "[after cleaning] then what?". I'm planning to conformal coat the board with Parylene after it has been cleaned (and tested). – Nick Alexeev Mar 12 '12 at 01:02
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Might it be less expensive to just attach amplifiers with low output impedance near the biological end of the EEG leads? Powered by itty-bitty rechargeable batteries? You would have to put the ends of the leads in a charger when they're not in use, but a least they wouldn't need cleaning or would be easier to clean. Hmm... I also wonder why they couldn't just be powered by a source that took measures to filter out noise from the source in your range of interest, or filtered it's own noise (choke?) out of the signal line. I'm just a youngun, so... yeah, please correct me if I'm wrong.

Mutant Platypus
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  • Yes, I'm aware of the existence of active electrodes and other helpful EEG practices. But, I'm looking for a cleaning procedure... specifically. – Nick Alexeev Mar 15 '12 at 02:09