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I need to get an old HC11 based EEG circuit at Ebay because I need to copy the firmware for my corrupted unit. It has had old NiCad batteries in it since 1999.

I just want to know if NiCad fluid leaks can destroy any circuit on the PCB. For example, if the HC11 chip is soaked in the fluid, will it still run if the fluids are cleaned (what solution do you use to clean it)? Any experiences?

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    \$\begingroup\$ Leaking NiCds sounds super toxic to me. \$\endgroup\$
    – DKNguyen
    Oct 10, 2022 at 22:33

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Most EEG circuits have a high impedance (over 1MΩ) if so any contamination would increase the conductivity across the surface of the PCB (an between traces) and make the signal noisy from leakage currents. Techspray (1612-10s) is usually a good solvent to clean most boards or isopropyl alcohol. You'll need to remove all of the contamination or you might not have good results.

Warning, nicad electrolyte is toxic and a heavy base (KOH). One problem you might have is if there is any corrosion, and if there is the circuit might be done for.

While cadmium is categorized as an acute toxic substance, nickel and nickel oxides fall under the category carcinogenic substances under inhalation [21]. NiCd batteries use a strong alkaline electrolyte (25%–30%) based on KOH as the main constituent. This type of electrolyte is extremely hazardous with respect to skin contact. A contact to mucous membranes, such as that of the human eye in particular, may end in irreversible damages. Special precautionary measures have to be taken in the production and when working with batteries for that reason.

Source: https://www.sciencedirect.com/topics/engineering/nickel-cadmium-battery

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  • \$\begingroup\$ For nicad batteries that are 20 yrs old. Do all leak or only some of them depending on moisture, ambient tempeature, etc? \$\endgroup\$
    – Jtl
    Oct 10, 2022 at 23:24
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    \$\begingroup\$ If the leak and corrosion started from within I would think it's more due to the battery construction and electrolyte, if it started from the outside it's probably due to humidity or something like that \$\endgroup\$
    – Voltage Spike
    Oct 10, 2022 at 23:31
  • \$\begingroup\$ I don't think all leak, only the ones that have been on trickle charge for 20 years, as for corrosion it thik it;s mosly electrolytic so if the device was unpowered there will be less damage. \$\endgroup\$ Oct 11, 2022 at 4:19
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    \$\begingroup\$ I think the term you want is "strong base", not "heavy base". \$\endgroup\$
    – Hearth
    Oct 11, 2022 at 5:55
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The electrolite in NiCads is KOH aka caustic potash, a strong base. It will attack copper, lead, and tin, among others, and your skin as well. Wear at least gloves and eye protection when dealing with KOH.

You can probably google some tricks to neutralise and remove the electrolite (I would start with a weak organic acid like vinegar or lemon juice and an old toothbrush, then rinse, and then apply the usual PCB cleaning agents, but please google to confirm).

I have no idea if individual ICs survive long exposure to KOH, but in case of leakage, the PCB and the solder connections will certainly have suffered, and possibly be beyond repair.

Or, maybe, the NiCads haven't leaked yet. A photo of the NiCads or PCB before you buy could tell you.

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The liquid in the NiCd cells is a pure potassium hydroxide and water. Neither Nickel nor Cadmium are dissolved in it in detectable amounts.

It is corrosive and it also can burn the skin (or eyes!), but not particularly poisonous.

Exposed to air, potassium hydroxide converts to potassium carbonate - it is somewhat less corrosive and generally not dangerous (but be careful anyway).

The substance that one can use to wash it away is the pure water. Distilled water is overkill, any drink-safe water is OK. Most printed circuit board materials survive short (few hours) exposition to water pretty well and the pure water cannot do more damage than the cell electrolyte already did anyway.

Soaking the board in water for an hour or two and/or using a brush are allowed approaches.

Be sure to dry the PCB very well (beware of water between the board and the elements) before energizing it.

Compressed air can be used for best results against the hidden droplets.

Washing it with ethanol (after the water) can accelerate drying pretty well, but ethanol can wash some paint from the PCB or the components and this is probably not what you want with a single item.

Quite often a well-cleaned PCB starts working "like new". Try this before any other attempts.

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