Aluminum electrolytic capacitors can and do fail over time; and are the most likely suspects in any given circuit that's more than 5 years old; particularly if the circuit is unused for extended periods of time.
The capacitors are basically made of two sheets of very thin aluminum foil that's rolled up, placed in a can-like package, and filled with an electrolyte fluid that is alkaline in pH.
Then the capacitors' dielectric is formed by very slowly charging the capacitor up to its' rated voltage via a current-limited source, until the leakage current through the capacitor falls to an acceptable level.
Some environments are particularly brutal; extreme temperatures, shock/vibration, corrosion, electrical noise all take their toll.
Even if an electrolytic capacitor is simply unused for an extended period of time, the dielectric will degrade; the longer it is not used, the worse the dielectric becomes. The capacitance is reduced, and the leakage rate increases. If the leakage rate becomes excessive, there will be enough power dissipation in the package to cause the electrolyte to boil, rupturing the package forcefully.
slawie
