Advice needed on leaky cap cleanup!

Zetr0 said:

okay here goes - heres the science bit - I tried to keep it from A level chemistry as possible without going into full mind-numbing and tongue swallowing reaction equations.

There would be better ways to say this but this is the laymen expression, if anyone feels a better expression or more detail is require please add to or post on =D

It is a bit winded this post so jump to the end of it to catch the highlights.



Still here? then Lets Begin =)


Whats is the NiCad Battery?

Rechargeable battery on the A500 is the same on the A2000 - it is a NiCad (Nickle Cadmium) Accumulator.

A NiCad Battery consists of nickel oxide hydroxide, and metallic cadmium as electrodes - this is then immersed in (potassium hydroxide [KOH]) an alkaline electrolyte.


When it leaks: Reaction Time

Initially what happens is the reaction process here starts off with with the battery leak - this causes the Potassium Hydroxide to react with the air.... this over a time and room temperature will create Potassium Perxoide. Both of these chemicals react quite strongly with Copper.

Peroxide is a VERY strong oxidizer, when you have this combined with Copper (an oxidizing metal) you have a very compound problem, as you can imagine what starts off small will crescendo to a larger problem in a short space of time.

Environmental factors do play a part in this, from temperature to air humidity - all effect this copper - potassium hydroxide / peroxide reaction.

Now potassium peroxide is an inorganic compound with the molecular formula K2O2, it is also a mild alkaline to a base pending on its strength.


How do we fix it?

To neutralize an alkaline we must use an acid: of course acid will also react with Copper, in fact there are very few things copper wont react with, so we need something safe and controllable - Acetic acid (i.e. spirit vinegar) - although this would react with copper - its relatively insignificant when compared to the peroxide base that has built up in concentrations on the PCB.

The acetic acid neutralizes the base and provides an alternative for oxidization with the peroxide being that its an organic compound. As a plus point it also breaks down the concentrated Copper Hydroxide (those blue/green fuzzy bunnies) left over from previous reactions.


But why?!?!?

Copper Hydroxide is mildly amphoteric, This is the double whammy for the copper tracks as the more that reacts the more alkaline is produced thus the more it reacts until it runs out of oxygen/copper..

There are four driving factors for this reaction

1. The amount of potassium hydroxide that leaks
2. The amount of potassium hydroxide that reacts to form potassium peroxide
3. The air humidity
4. The temperature of the PCB and environment.

I could go on with the forming of crystal and copper dendrites and electrical impedance - but that's only going to bore the pants off every one lol


In short (for those that skipped)

1. A NiCad battery has an alkaline electrolyte.
2. Electrolyte leaks and reacts with air to form a strong oxidizer
3. Copper is an verilent Oxidizing metal
4. Electrolyte is an Alkaline and will concentrate to a Base
5. Copper reacts with Oxidizer causing more Alkaline
6. Alkaline reacts with Copper and Air creating more Alkaline

Once the battery leaks its like lighting a fuse to a firework and running away.

Click to expand...