Why do capacitors leak?

[Unknown_K]

Those old SMT liquid filled capacitors are kept plugged up by cheap rubber that degrades over time so they leak. The capacitors in the plague era were made with unstable compounds that expanded and leaked from over presssure.

 

[LazarusNine]

Apologies for resurrecting this thread, but it appeared in my recommended 'similar threads' list. I'm currently waiting for a shipment of caps to fix the PSU in a Power Mac G4 MDD, and I speculate that the biggest contributor to failure in PSUs is heat. The motherboard caps still seem fine, but they'll need replacing eventually. I've taken the opportunity to buy replacement fans for the "wind tunnel" PSU, but I've gone with ones with an even higher CFM rating than the stock fans, so I suspect noise reduction will be minimal. It's always a bit of a let down when you have to replace electrolytic caps with electrolytic caps knowing full well that leakage will eventually be an issue again. I guess, in death, I won't care about cap leakage.

 

[Phipli]

but I've gone with ones with an even higher CFM rating than the stock fans, so I suspect noise reduction will be minimal

Take care with replacing fans based on just CFM - CFM is usually quoted for the fan operating in an open environment with no back pressure. It doesn't account for how the fan reacts to back pressure and Mac fans do more work than regular PC case fans as they usually are pushing through the PSU, the case, and out again, and there is only one or two.

Think of it like a sports car and a tractor - both might be 300hp, but one uses it to go fast, the other uses it to drag a plough through a rocky field. CFM is sort of like quoting straight line speed.

 

[ArmorAlley]

An unused motherboard in a vacuum-sealed bag, and stored in a dark place at room temperature, will be quite well protected from all four of those destroyers. That's not a guarantee the capacitors will still be good after 40 years, since the electrolyte may also react with the seal or other components. I don't think you can stop cap rot forever. But if I were a betting man, I'd bet that sealed NOS motherboards would be in substantially better shape than other identical motherboards the same age. Thoughts?

My first thought about such motherboards is whether there is a battery in situ, and, if so, has it shed its load?