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Illicit Leaky Capacitors Killing Motherboards
mcd7756 writes "The IEEE Spectrum magazine has an article about how capacitors made with a stolen formula for the electrolyte are leaking and causing motherboards to fail. Some computer manufacturers are admitting to the problem; others are hiding it."
http://slashdot.org/article.pl?sid=02/11/04/175121 0&mode=flat
Basically anything that uses a switching power supply or a switching voltage regulator is at risk IMO. That covers most consumer and commercial equipment other than stuff that's physically too small to have one of these caps, or that handles only audio/radio.
The issue with electrolytic capacitors is this:
All capacitors have what is called an equivelent series resistance, or ESR (great: now we have 2 TLAs that are overloaded in context: RMS and ESR... )
The ESR only matters when there is a current flow through the device - a static voltage does not create (much of ) a static current flow - that is rather the definition of a capacitor. So if you are putting a DC voltage across the device all is well.
However, if what you are putting across the device is NOT DC, but rather DC with an AC component on top of it, then there will be a current flow as the capacitor tries to hold the voltage constant (again, that's rather the point).
However, due to the ESR, some of that current will cause heating of the device (power = I*I*R).
In caps with the good electrolytic, nothing much happens. In caps with the bad electrolytic, the electrolytic breaks down into hydrogen and oxygen, as well as boiling into steam. Pressure builds, and eventually the cap leaks. Since the other stuff in the electrolytic is caustic, your PC board traces rot away.
Now, at low ripple currents, this does not happen very fast, and any cap will have a long lifespan. However, as you approach the limit of the cap, the heating becomes the dominant factor, and the cap will cook itself fairly quickly.
That's where that 4000 hours comes from - that is not the cap running with a few tens of milliamps of current ripple across it, that is the cap getting amperes of current rammed down its throat, and running very hot.
That's also why you use tantalum caps wherever possible - tant's don't have an electrolyte, they use a very spongy tantalum slug with lots of surface area. They don't have quite the capacitance per unit volume that electrolytics have, but they don't leak, either. (but they do blow up real good (sic) when you exceed their rated voltage!)
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