I've got a design that I've inherited with a pretty standard n-channel mosfet driving a relay that controls a motor and actuator.
On a recent build we started getting a 50% failure rate on the n-channel mosfet. Previously we had no failures of the mosfet. The only differences I've been able to find so far are different date codes on the relay and mosfet. Otherwise nothing has changed.
The mosfet is an ON Semiconductor 2N7002LT1G
The relay is a Omron Electronics G6RL-1-ASI-DC24
The flyback diode is a ON semiconductor MRA4003T3G
The mosfet was examined by ON semiconductor and it was found that it was most likely destroyed by excessive voltage. But I've not been able see a voltage spike on the mosfet above 30V so far.
Here is the part of the circuit with the mosfet/relay/diode.