Why does water short out PCBs? (i.e. Why doesn't electricity follow the path of least resistance?)

Question

If copper is more conductive than water (at any reasonable PH), submerging copper electronic circuits in should have no effect, as the electricity should continue to follow the path of least resistance (the highly conductive copper PCB paths, for example) rather than shorting into the mildly conductive water. However, dumping water on electronics clearly shorts them out, even though copper is the superior conductor.

Why does this occur when the path of least resistance should be the copper rather than the water?

Answer 1

Added 2024: I consider the answer below to be clear, but based on a comment added (and a downvote), apparently not clear enough for some. I decided to add the following. This is a summary of the answer below.

• For a given DC voltage and given "multiple effective resistance" paths the current will follow ALL available paths, with the current dividing in the inverse resistances of the paths. For example, if 10 V is applied and there is a 20 Ohm and a 1 million Ohm path in parallel then :

  • The 20 Ohm path will carry I = V/R = 10/20 =0.5A.

  • The one million Ohm path will carry I = V/R (again) = 10/1,000,000 = 10 uA.

• HOWEVER, unless the water is completely pure some ionisation will occur, which will usually increase conductivity and so increase current flow so that even more ionisation will occur so that ... . To what extent the process "runs away" depends on the nature of the impurities and environmental conditions.

• In practice, an electronic device immersed in tap water with power applied will generally produce enough ionisation products to allow secondary electrolysis of components, increasing ionisation products and after a while, device destruction.

Long ago I found camera in a forest park. I dried it and on returning home after several days placed it in a warming cupboard and informed the police. It was recovered by the owner but was irepairable.

A friend dropped an iPhone in a toilet. It died.

I dropped a pager (receive only text message device) into salt water and a cellphone into pool chlorine solution - in both cases I promptly removed the battery and washed the device in copious fresh water.

Both survived.

______________ Original below here __________________

Fallacy correction: "Electricity" NEVER 'follows the path of least resistance'.

Rather, it will follow all available paths in inverse proportion to their resistances. *

Electricity is NOT like a single large Zorb bounding down a hillside.

Electricity would be better modelled in this context as
a large reservoir of water being poured down a rough hillside.
Most of the water will follow major gullies and channels but some will find smaller side channels.

The question is not
"which path will the electricity follow?"
but
"how much current will flow along this given path?"

Low resistance paths will conduct higher currents.

Higher resistance paths will conduct less current.
Only infinite resistance paths will conduct zero current.

There are NO infinite resistance paths.

Plus:

When voltage is applied to water above a certain critical voltage, the water will decompose into Hydrogen and Oxygen - - known as "electrolysis". Any components in the water are liable to also decompose to produce eg Chlorine gas from chlorine products in the water. Even wholly pure deionised water can be decomposed in this manner.

Once you get even a small amount of current flow you get ions formed which promotes more current flow, lower resistance, more breakdown ... zap.

I have dropped a "pager" into sea water and a portable phone into concentrated chlorine solution* and "saved" both with no long term damage by immediately taking out the batteries and washing them in copious quantities of fresh water and then leaving them to dry completely (a very important step). And I have seen equipment destroyed by exposure to water with the batteries then left in.

• Note to self: Remove portable phone from top pocket before stirring large bucket of Sodium Hypochlorite solution.

When splashing along sea edge and generally having fun your pager should be in a salt-spray protected bag or, better still, left at home.

Note to the young: Pager's were things we used to have before they invented cell phones. Effectively a receive only SMS system with no voice or any other features. Doctor's still seem to use them.

---

• I have used the term resistance for paths that current follows. This works well for DC voltage and the paths' resistive components. Using AC voltages and paths with reactive components increases complexity, but, the same principles apply.

Answer 2

The problem is that although the traces have a very low resistance, the traces are not a fully meshed network. The PCB is a collection of networks. There isn't a low resistance path between any two nodes, but only between specific nodes which are understood to be on the same network, as dictated by the circuit design which was transferred to the PCB artwork.

Between some other nodes that are not on the same network, the resistance or impedance may be, in fact, very high. Close to infinite, in some cases. Moreover, there may be a voltage between these points. A prime example of this are power rail networks.

Ion-rich water will create conductive paths between nodes which are not supposed to be thus connected, and the voltage difference will cause a current to flow, creating short circuits or near short circuits.

Answer 3

A circuit board has traces on it. These traces are meant to be insulated from one another. Proper circuit operation requires it.

If you dump water on the board, the traces are no longer insulated.

If the "circuit" was just a solid plane of copper, the water would have no effect.

Answer 4

It is following the path of least resistance. In this case, that path is the copper and the ions in the water that allow some more electrons. You need to look at water as a very high resistance. It isn't infinitely high, so some current will flow there, usually with bad results. Just as in the case with two resistors in parallel, the copper is the low resistance, and most current flows through that path, but some will flow in the other high resistor (ions in the water). Note that deionized water cannot conduct due to the lack of ions.

Another thing usually ignored is that ICs are not completely hermetically sealed, so water and other elements can go in and wreak havoc inside, especially with time when you take into account oxidation and ions that can short the IC.