Does water leave any damage after drying out?
If it does, where and how exactly does it? How can it be repaired?
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There are three ways water can damage components:
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).
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.
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.
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 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.
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.
Much of the damage caused by water is done while both water and electrical current are present, but you have asked about after the water is gone.
In theory, no, dried up water shouldn't harm electronic components as any H2O will have evaporated, dried up, etc. However, theory and reality are not the same. In reality, the vast majority of water contains some sort of particles and it is these particles that can cause damage to the electronics after the fact. Depending on the quantity of the particles, the type of particles and the environmental factors the system experiences, you can run into a variety of problems.
If you are talking about spilling a glass of drinking water on a circuit once, you probably don't need to worry about these particles and should just focus on removing power ASAP and drying out the system as much as possible. If, on the other hand, you are talking about river water, soda, repeated exposure to any liquid, etc., you should be concerned about these particles.
Please note, water can also cause non-electrical issues that may or may not impact the functionality of the entire system.
During my freshman year of college, my roommate dropped a small carton of milk on her laptop keyboard. After disconnecting power and the battery, I consulted my father who suggested giving the laptop a bath and letting it dry off for several days. Unfortunately, the laptop wouldn't fit in the sink so I took it in the shower and did what I could to rinse the milk out of the electronics. We then set up the laptop on its side with a fan for a week before putting the battery back in and turning it on.
She got maybe three more months of life out of that laptop. This was long enough to recover her data, but not long enough to make it through the school year. The water hadn't caused any additional damage to the circuitry, but the particles left by the milk (and, realistically, some left by the water) caused corrosion on the metal wires and solder joints, eventually eating them away enough that they could not effectively pass electrical current.
The particles left by milk are basic (as in the opposite of acidic, not the opposite of complex) and can therefore react with the elements that make up electrical circuits. Acidic particles would have a similar result. Often, water contains some sort of salt (not always NaCl); salts (given enough humidity/moisture) can carry electrical current and cause shorts within the circuitry. There are probably several other realistic categories of particles that could cause damage to electrical circuitry.
If at all possible, disconnect the damaged part of the system and work with that part alone to avoid causing additional electrical damage. This is often not an option with electronics that have been purchased.
Before applying power back to the circuit,
After applying power back into the circuit if there is still damage,
Another point I haven't seen mentioned that water can cause is thermal stress.
I've seen two more or less separate forms of this. Both (in case it wasn't obvious) apply primarily to hot-running circuits.
If you have an electronic device submerged in water , First remove any power source, if it is an unsealed battery discard it. Rinse in de-ionised or distilled water then dry it.
Damage done depends upon type of water and electrolyte content and weather a power source was available, and time of exposure
Drying can be done with rice according to the telly could be done with rice.
20 years ago we used something called Fomblin which later turned out to be a bad idea for water displacement.
Better to use elevated temperatures and a dessiccant such as 3A zeolite or preferably a vacuum chamber. Early cell phones had LCD displays which had no edge seal and could me easily damaged.. Other components such as electrolytic capacitors should be replaced.
EDIT: Older electrolytics sometimes had a vent (wire type) and microphones had a protective cover during manufacture , these had to be replaced as well. I do not know the current recommendations for more modern devices. Water displacement used to be recommended but i do not know what is the best for this these days.