Does water actually damage or only temporarily disable electronics until drying out?
Water can damage electronics in a few ways.
Most people only think of damage due to the conductivity of water. This means that the water will short out the electronics (and this is why water and mains voltages are dangerous.
However, with most lower-voltage electronics, the currents that flow through the water are not that substantial and doesn't do a lot of damage (though it can!).
What is often the cause of damage with electronics exposed to water is corrosion. The water will act as an electrolyte and as such will dissolve and corrode away many of the tiny tracks and pins on components and circuit boards. If this is the case, it is sometimes possible to repair this after the fact by reconnecting the correct parts.
This is why removing the battery from a phone that has been exposed to water can often save the device. With the source of power removed, no more current can flow through the parts exposed to water and thus the rate of corrosion drops.
There are three ways water can damage components:
- Damage inside IC packages
- Corrosion
- Shorting or changing impedance.
IC packages
There are a lot of ICs that have a specified humidity tolerance, partially because if they do get water in them they will cease to function. Devices that are more affected by this are accelerometers and other MEMS devices, and some optical devices. Most chips are sealed to some extent, but water entering into them can create shorts and once the water is inside it is difficult to remove.
However, if you don't have moisture-sensitive parts, you can actually use water to clean PCBs and ICs. This is actually how water soluble solder works: the flux is water soluble and rises away (which is how I prototype products at work, but also using deionized water). Most PCB assembly processes now use no-clean solder, which is supposed to be non-polar and unaffected by water (but who really knows).
Corrosion
Water can activate salts and other materials on a PCB which can corrode metals (and essentially turns traces into a battery). Any water will dissolve contaminants and create problems, or leave residues even after it has evaporated. Any ions in water (especially salt water) will react with metal and contribute to corrosion.
Shorting or changing impedance
Pure water is not conductive, but this changes quickly as it picks up contaminants. As soon as it starts to be conductive there is no more control over where currents flow on a PCB, and they will take the shortest path through the water back to the source. This will wreck havoc with the power supply and any electronics sensitive to overvoltage. Even small amounts of water will change the capacitance around traces and cause problems for high speed signals.
You can actually run a computer for a short time in pure water, but after it begins to conduct (because the water picks up and dissolves contaminates, which makes it conductive) the computer will lock up and then short out.
Repair
The first thing you ever do is remove power to the product, whether it's the battery or a power supply.
The second thing would be to remove all remaining water. This can be done with heat (not hot enough to damage components), desiccant packs (which actually works well) and a lot of time.
If you really wanted to go to extremes, placing the device in a vacuum (after removing batteries and other vacuum incompatible devices in a product) would remove any volatile gases or liquids such as water.
Use flux remover and remove any residues that may be left on the board. Check components' leads for any damage or corrosion. Any damaged parts will need to be replaced. Check the datasheets for all parts: any parts that are MSL (Moisture Sensitivity Level) level 3 will need to be replaced.
Odds are the battery has shorted and will need to be replaced.
I have had success with phones and desiccant packs and a day or two of drying while applying a little heat (about 80 °C).
Water can create a conductive path between two circuit elements (e.g. traces on a circuit board, pins on a chip, etc.) that has much less electrical resistance than the intended path. Therefore, when the same voltage is applied by the power source, the current can be much greater (Ohm's law).
This higher current creates more heat and can burn parts, including tiny parts inside chips. If this has happened, recovery is not possible without replacing any parts that have been so damaged, much like how fuses need to be replaced rather than repaired.
If water created unintended current paths but none produced short circuits or burned out any components, it might have very strange and unpredictable behavior that could be temporary, until it dries out, because signals are going places they weren't meant to go. If the device contains motors, heating elements, etc. it could physically damage itself (and/or its surroundings) based on these erroneous signal paths. Turn it off and remove power sources until everything is dried out to help prevent such an outcome.
During the time when the water was creating lots of extra circuit connections, onboard memory (state machines etc. programmed in) might get into a strange state with inconsistent or unexpected data. In this case, a reset might be required. Some devices have a tiny reset button that you can poke with a pin or toothpick for such purposes.
Removing any power source (including stored energy in batteries, capacitors, etc.) prior to water exposure can help avoid damage from shorts, and is a good idea if an elevated probability of water exposure is anticipated. If the device is already off prior to water exposure, don't turn it back on immediately after the water exposure (e.g. "to see if it works") but instead remove power sources and wait for it to dry out!
Water can also make some (esp. pulp-based) physical materials more flexible and malleable, making the parts more likely to be damaged by physical motions that would otherwise cause no damage. Water can also dissolve some glues that hold parts together.
Water can also dissolve some materials and carry ions away, and a bit of corrosion can be a big deal for a tiny component.