Failure Modes of Capacitors in Computer Applications
We will look at the physical failure modes of capacitors later, basically all the bulging and leaking but it is important to understand what is going on with the performance of the capacitor when it is considered to have failed. Basically the capacitor is no longer performing to its normal specification.
Teapo SH Series Cap that was filtering the 12v output of a power supply
Bulging and showing very high ESR of 9 ohms
It is important that the capacitors used on computer motherboards or other devices have Low ESR (Equivalent Series Resistance). The most typical failure mode of a capacitor is that the ESR has become quite high and therefore it cannot be considered to be a Low ESR capacitor anymore. This can be tested with an ESR meter which is a very useful device for a technician to have in his toolbox.
For a motherboard we would want a capacitor with ESR of ideally 0.05 ohm to 0.01 ohms or less, especially on VRM output. A failed capacitor could measure say 0.20 ohms through 1.5 ohms, 2.5 ohms and all the way up to the highest I have measured which is the teapo in the photograph which is measuring 9.0 ohms!! Typically a failed capacitor will have become many times it's normal ESR. Therefore failed ones are easy to spot.
It is not really necessary to start identifying the capacitor and finding it's specification document. If you know that a Low ESR capacitor should be used in that location then anything like 1.0 ohms or higher is simply awful. For Low ESR applications you really want a capacitor that is as much lower than 0.10 ohm as you can find. So replace any cap which is measuring over 0.10 ohm otherwise it will eventually damage other capacitors in the circuit even good ones..
High ESR is basically the same as putting a resistor in series with a capacitor. The circuit will not perform properly and will have increased noise.
A capacitor with High ESR will probably be bulging although that is not always the case.
Another failure mode of capacitors is that the capacitor tested is measuring acceptable ESR but that its capacitance is much lower than it should be. So it is useful to also have a capacitance meter to hand in order to measure the capacitance and see if this is the problem.
A capacitor can fail open so that it is disturbing the flow of electricity in the circuit and then the circuit is no longer working at all. Polymer capacitors are supposed to fail in this manner and it is much preferable than failing shorted as discussed below. However they can also fail shorted unfortunately. It is possible for a capacitor to fail open without any visible signs.
A capacitor can fail shorted which is probably the worst thing that could happen. This is one of the reasons why it is important to replace capacitors which are failing early. A shorted capacitor would probably damage the motherboard to an extent that it would be impossible to easily repair requiring components to be taken from a donor board and used as replacements. It is possible for a capacitor to fail shorted also with no visible signs.