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I apologize in advance if this question is off the topics answered on this forum. Some hours back my drone fell into some dirty water. I was able to rescue the device, unplug its battery, open up its circuit board, dry it thoroughly using a hairdryer and finally putting it in place. It worked fine afterwards. But my question is how I could prevent or rather reduce the chances of the circuit board been corroded soon?

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    \$\begingroup\$ A wash in clean water or de-ionized water before drying would be the only additional step in this procedure. And a very, very complete drying time, checking that water does not remain trapped between components and printed circuit board, or remains hidden inside connectors. \$\endgroup\$ – glen_geek Nov 25 '17 at 16:35
  • \$\begingroup\$ I must add that a bath of distilled water in an ultrasonic cleaner will do wonders. \$\endgroup\$ – S.s. Nov 26 '17 at 1:38
  • \$\begingroup\$ please change your title to prevent corrosion of circuit board fallen in water or something like it ... it describes your situation better \$\endgroup\$ – jsotola Nov 26 '17 at 5:49
  • \$\begingroup\$ Acer and Apple laptops of mine both corroded extremely fast from rain. ( acidic no clean flux... argh) Yet I have fixed others with 2 bottles of isoprop, vacuum, then 1-2 day warm forced air flow dry \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 26 '17 at 23:26
  • \$\begingroup\$ For corrosion prevention and improve electrical insulation from HV breakdown, I use WD40. It is derived from petroleum which tends towards a dielectric constant like that of PCB material.~4 which adds capacitance of crosstalk to exposed tracks to nearby conductors. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 26 '17 at 23:37
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You need to wash it.

Seriously, what will cause corrosion is any salts that were dissolved in the dirty water that have remained behind when you air dried it.

You need to buy some distilled water and repeatedly wash the board a few times in fresh distilled water to dissolve and remove as many of those salts as possible. Flushing under components may require the use of something like an ear syringe, or as Peufeu suggests, a dental water jet.

Then, once the thing has had sufficient time to dry, and if there is a risk of this happening again, you should coat the board in a urethane, or better.. epoxy, so this will not happen again.

TIP: It is an old trick to stick your board inside a bag with a desiccant for a few days to completely remove any moisture trapped inside hidden places on your board, wires and other components. Rice apparently, despite the rumours, is not a great choice and there are better desiccants just don't use a salt, and keep the desiccant itself from directly touching the parts.

NOTE: Salts + humidity = electrolyte. Not only will that corrode stuff, but it also conducts which can cause shorts, arcing, and function/part failure.

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    \$\begingroup\$ Rice does not work. Use an oven or a real desiccant. \$\endgroup\$ – mkeith Nov 25 '17 at 17:23
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    \$\begingroup\$ @mkeith nvm I retracted it anyway... it was not really part of the answer in the first place. \$\endgroup\$ – Trevor_G Nov 25 '17 at 17:38
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    \$\begingroup\$ @mkeith ya I can see that normal rice is pretty much useless. converted rice.. maybe. Anyhoo, onwards and sideways. \$\endgroup\$ – Trevor_G Nov 25 '17 at 17:47
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    \$\begingroup\$ Save silica gel packets - they're a great desiccant and you can easily regenerate them once they saturate just by putting them in the oven at 120C (250F) for an hour or two. \$\endgroup\$ – J... Nov 25 '17 at 20:50
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    \$\begingroup\$ @J...: You can also cheaply buy silica gel in bags of several kg in any well stocked supermarket or pet store; it's sold as "crystal cat litter". \$\endgroup\$ – Ilmari Karonen Nov 25 '17 at 23:08
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If there are still ions (salts) on your board, any humidity which condenses on your board will become conductive, and corrode the copper away through electrolysis.

My favorite tool for washing a board is a dental water jet. It's great for getting junk out from below SMD chips. If you don't own one, you can also use a spray with the nozzle set to make a jet, but it is a bit more exercise.

If you use tap water, there will still be a few ions on the board, though. So it would be best to use distilled or demineralized water. As an alternative, you can use compressed air (or canned air) to blow most of the water off. You can also spray some isopropyl alcohol to blow the water off, but this wastes a lot of it.

Make sure you also look at the connectors and motors in your drone, unplug all connectors so the contacts dry, etc.

The hair dryer is nice because it will blow the water off too, and dry the hard to reach places, but you don't need to dry things ASAP. A circuit can stay wet for a while without anything bad happening if it is not powered. So rushing to unplug the battery probably saved your board.

You can use spray-on conformal coating on the board to protect it from water. This is easiest to remove compared to epoxy, if need be. However, be careful about the connectors... the stuff is insulating, so if it gets into contacts, potentiometers and the like, you're in trouble.

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    \$\begingroup\$ +1 for dental water jet. I would not use the tap water round here... water is way too hard. \$\endgroup\$ – Trevor_G Nov 25 '17 at 16:53
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It is so called conformal coating. High end electronics for industrial, automotive, ...use are coated with a varnish. Could be based on epoxy, silicone and other based upon the final use.

You should wash the circuit in isopropyl alcohol and then dry it, as the water can contain salts.

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    \$\begingroup\$ isopropyl alcohol wont dissolve salts too well though on it's own. Diluted with distilled water it may. \$\endgroup\$ – Trevor_G Nov 25 '17 at 16:39
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    \$\begingroup\$ Unless you have access to very pure isopropyl alcohol, I definitely recommend following the alcohol rinse with a distilled water rinse, then dry. The isopropyl alcohol I have used (from pharmacy or hardware store) definitely leaves a resisdue. \$\endgroup\$ – mkeith Nov 25 '17 at 17:22
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    \$\begingroup\$ @mkeith, Yes, the purity of the isopropyl alcohol matters. I wouldn't do it with anything less than 90% (commonly available), with a preference of 99% (available to consumers from some common stores). As Trevor mentioned, isopropyl alcohol and water dissolve different things. For things already imersed in water, I'd do washes with distilled/demineralized water, then one or more washes/rinses with 99% isopropyl alcohol. \$\endgroup\$ – Makyen Nov 25 '17 at 20:02
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    \$\begingroup\$ When using isopropyl alcohol, a considerable amount of noticeable residue will be the result of dissolving material already on the board and concentrating it in noticeable patterns, not actually from the isopropyl alcohol itself. This is commonly the case with various "no residue" fluxes. This can be solved by using enough isopropyl alcohol to actually flush the material off the board and/or wiping up the wet areas. \$\endgroup\$ – Makyen Nov 25 '17 at 20:02
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If you don't want to coat your stuff in a hard potting compound (epoxy) or conformal coating (acrylic) then you can smear/immerse it in dielectric grease.

Amazon has something like this readily available, But really, any "waterproof marine electronics grease" will work.

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