Partial load failure detection circuit

I think there may be two reasons. First, here's a transistor that is specified for use in ignition systems and note that it has a protection circuit built in that will turn the transistor back on (thus protecting itself) if the voltage at the collector exceeds 350V.

Normally, car ignitions won't generate much more than a 300V spike and to demonstrate this here's another picture taken from this site: -

That site also explains something else which may have resulted in the failure of the IGBT. Dwell angle is the time period that the contacts are closed before opening to "generate" the spark. On the diagram above this is about 3ms (note the lowest part of the trace just before "firing". In this time period, the current in the coil (from the battery) builds up to about 8A - this 8A is deemed the right amount of current to generate the correct amount of energy to produce a decent spark.

If you doubled your dwell-time (ignoring coil resistance) you'd get 16A - it's a time-linear thing and if of course your points breaker was just an old-fashioned breaker that could take a gazillion amps it wouldn't care much about dwell angle and this means you've probably exceeded the current rating of the IGBT and it's fried without you knowing about it.

Here is an interesting reference article to building your own car ignition using a 555 timer - it, I suspect sets the dwell angle.

Two immediate modes of failure are over-voltage and over-current.

1. If you have a high impedance input like a gate to a mosfet, then high voltage (even at very low current) will cause a puncture in the capacitive gate of the mosfet as the electrons have enough energy to cause the dielectric to breakdown. Once this occurs, the resistance of the input drops to near nothing and a later low-voltage high-current will further heat up and destroy the mosfet. This mode of failure is why there is ESD protection on many chips.
2. Over-current causes over heating of the device. Once temperatures get high enough to start changing the structure and/or burning of the internal semiconductors, it will start acting funny, working less efficiently, or completely failing as an open or short.

It's possible that you could think of reverse voltage as another failure mechanism, but generally that still falls under one of the other two categories, it's just different to think about. For instance, if someone reverses a power supply on a circuit with a diode in it, they may expect no current through the diode and instead get an over-current condition because the diode would now be forward biased.

Note that capacitors, resistors and inductors (and any other circuit element) are likely to be damaged in similar ways as transistor ICs, i.e. over-current and/or over-voltage.

Other failure modes of electronics in general may be found here: http://en.wikipedia.org/wiki/Failure_modes_of_electronics