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What happens when a ceramic bypass capacitor fails? I am picking out parts and is wondering is it worth the extra few cents for rated capacitors?
Since generally most IC can function without a bypass, if a bypass cap fails there should be no problem, right? Or when a ceramic bypass fails does it form a short circuit?
Surface Mount Multi-Layer Ceramic Capacitors (MLCC) primarily fail in the cracking regime.
That is the ultimate failure mode of a capacitor is with a body crack.
MLCC are made of very fine intermeshing metallic layers embedded in a ceramic substrate.
A large portion of these failures result in short circuits (low impedance path) through the part. The severity of this outcome in a decoupling situation cannot be understated. The ceramic can take a lot of power and I have seen even 0402 shorts take out entire PCBs sustaining multiple amps through them.
Occasionally the component fails in such way that a short is not produced and simply the capacitance is dramatically reduced. This is not the usual failure mode and is largely a matter of luck and failure trigger (sudden crack will fail open more often than a gradually propagating crack).
The variables that contribute to component failure have dozens of possible sources, vibration, electrical stress (ripple), dielectric type, temperature, PCB placement, reflow temperature, depaneling process component size (plate density)... Component quality also plays a role because substrate voids and hot spots can concentrate stresses.
In principle the component should last a very long time in a stress-minimized environment with good PCB and electrical design, and high quality MFG.
NP0/C0G parts are particularly suitable for reclaiming due to their much better dielectric (no piezo effect) and temperature (no capacitance change) characteristic.
Ripple and overvoltage are issues, but MLCC are typically rated for nearly 100% overvoltage and significant ripple. The concerns there are largely mechanical due to piezo effect.
Failure of one capacitor in a decoupling bank may further degrade others and cause cascade failures.
Ceramic capacitors can fail in a couple of ways.
They can be mechanically damaged - too much physical stress (pressure on the part or the board is bent a little too much) can cause a crack. The capacitor will then develop short circuits between layers. It acts more like a resistor in that case. If you are really unlucky, enough layers will short that the whole thing acts like a short circuit. Depending on how much current the power supply can deliver, it will either go "bang" or just sit there and overheat.
The other way they can fail is through over voltage. Too much voltage can cause the dielectric (insulating) layers to break through and become short circuits. Too much voltage can also cause the capacitor to flex, resulting in a stress crack. Then you get the same result as above. The capacitor becomes more like a resistor, or shorts out entirely - bang or burn.
Neither failure mode is usually a problem. Stay under the rated voltage, don't bend your PCB or hammer on the capacitors and you should be fine.
