I see a lot of CPU boards where battery damage is not being treated properly and as a result the alkali continues to eat away at the traces on the board. Eventually it eats through a trace, the board fails and it's sent out for repair (often after being damaged further in an attempt to repair it). This is a preventable issue which has been turned into a major issue. It is important to note that a lot of board repair guys won't take boards with major alkaline damage since it takes more time to repair than the board is typically worth.
Friendly Reminder: Get the batteries off the CPU board!
Note: While no boards were damaged in the making of this article, stunt doubles where used in some cases. In other words, there are several different WPC CPU boards used as examples.
The stuff (technical term) that leaks from alkaline batteries is potassium hydroxide, which is a very strong base with a pH of 12. It absorbs carbon dioxide from the air to form a feathery crystalline structure of potassium carbonate (11.5 pH). Before it transitions to potassium carbonate it is clear, so just because you don't see any white crystals doesn't mean there isn't potassium hydroxide on the board.
Potassium hydroxide/carbonate is like a cancer in that it will expand and migrate across the board where it oxidizes copper traces and components leading to permanent circuit damage. In the image below you can see where either through capillary action or electrochemical migration it has gotten under the solder mask (the dark areas indicate corrosion) and is eating away at the copper.
In the image below the alkali has completely eaten through the last 1/4" of the ground plane and the next portion is falling apart. You can also see where the trace has lifted from the board due to the alkali destroying the adhesive holding it to the board. Also note the corrosion on the inductor and the bubbling in the trace at the top of the resistor to its left.