Why does water damage low-voltage microelectronics so easily?

Question

Everybody has had the time where water gets all over their electronics, and the whole thing gets ruined, like cell phones.

At that low of a voltage (3-5 volts), I don't understand why it is such a short-term harm (long term makes sense - corrosion, etc.)

If an LED was in parallel with water, maybe a bit more current would be drawn, but it doesn't seem to be enough at all to short out the system, and the LED would still shine.

So what is it that permanently damages some electronics and what is the cause?

Answer 1

Pure water is actually not bad for electronics. Pure water does not conduct electricity. I've seen whole PCBs submerged in pure water and they run just fine. The problem is that pure water does not stay pure for long. It will quickly dissolve/absorb various contaminants from the environment, and those contaminants will cause the now not pure water to conduct electricity.

These contaminants come from the environment-- including the air. So dust, dirt, and even CO2 will cause the water to conduct. Tap water has lots of minerals and salts in it that will also conduct.

But normal water (not pure water) won't destroy most electronics when the circuit is off. I frequently rinse off PCB's in the sink, or even a normal dishwasher, to clean it. I just have to make sure that the water completely dries and doesn't leave a residue before turning it on.

But the reason why normal circuits, submerged in normal water, doesn't work is because normal water is conductive. It isn't a perfect conductor, but it is enough of a conductor. If you get enough electricity flowing in/through places that it wasn't intended to then that's bad. If you're lucky it will just make the circuit temporarily misbehave. If you're not lucky then you'll have permanent damage.

Simple circuits, like an LED+Resistor+Battery will likely work just fine when submerged. The LED might not stay lit, and the battery might be completely discharged. But dry it off and replace the battery and it should work fine. But some circuits are more sensitive. Think of a MOSFET that is switching hundreds of amps/volts. It takes only a little bit of electricity to turn the MOSFET on, and the water is just conductive enough to cause it to turn on. But now you have huge amounts of power turned on when it shouldn't be-- so it is no surprise that something can be damaged.

Or think about the resistive voltage divider on the feedback of a DC/DC converter. That is what sets the output voltage. Add some water and the output voltage could be forced too high. It wouldn't take much for water to mess up that divider. Now, instead of outputting 3.3v it is spitting out 9v. Of course, any chip that is being powered from 9v instead of 3.3v is probably dead.

So, non-pure water is bad. It kills things.

Answer 2

Although I have never heard of this happening, it is also possible that powering-up a water soaked device containing a cheap SRBP board could be a fire risk.

As a reckless teenager I used to take delight in wiring 12V dc across a pair of adjacent tracks on a piece of cheap stripboard, then placing a drop of tap water across the tracks. At first, all you get is a load of hydrogen & oxygen but eventually the heated water partly evaporates and partly soaks into the cheap SRBP base material. Eventually, the board gets so hot it starts to carbonize, then sparks appear between the tracks and ultimately the board catches fire!

I don't know what the minimum voltage would be for this to occur (I haven't tried recently!) but 12V at a few hundred milliamps will do it with 0.1" pitch Veroboard.

I have a disused in-car mobile phone charger which would be an ideal candidate for an experiment later ...

Answer 3

Low-voltage microelectronics often have low tolerances to higher currents and voltages. It's the nature of improved micronization and energy efficiency. By adding water, you are adding electrical paths for various parts that were not meant to be there. Things get shorted out, protection parts get skipped, parts can receive higher voltages than they can tolerate.

A given device might be powered by a 3.7 V battery or 5 V USB connection, but there could be step-up regulators for certain subsections of its electronics. You might have a device with a step-up to 18 V. Add water to create an unwanted electrical path, and that 18 V subsection just shorted out to a 5 V-only section, killing every chip on there.

An IC could only support sinking or sourcing 10 mA. Add water and a short to ground or V_cc would cause a lot more than 10 mA to be pulled, frying that IC's pin, if not the entire chip. Poof, there goes the LCD on your phone.

The main reason this happens is because it is not individual, unpowered parts, but an entire board of possibly thousand parts, all with various maximum voltage and current thresholds that are carefully laid out in a manner where electrical paths are carefully controlled.

To compare, your car can sit in the rain without water getting in (if it's well designed and maintained, naturally). Drive it into a river (or the river comes to you), and water will destroy the inside of the cab and engine. That's what you are doing when you introduce water to electronics.