Hi a snubber circuit is designed to reduce the high voltages experienced by power transistors when switching inductive loads.

If the snubber circuit fails the the transistor will fail soon afterwards. The high voltage (or back emf) experienced is proportional to the size of the inductive load (L), and the speed at which the transistor switches (di/dt). The formula is e =-Ldi/dt. The - sign represents the fact the the voltage is reversed across the transistor. The physical phenomenon is a property of the inductor (the motor in this case). An electromagnetic field is set up when a current is passed through an inductor, When the current is switched off the inductor tries to maintain this field hence producing the back emf (electromotive force).

By placing a normally reverse biased diode (snubber circuit) across the transistor (collector emitter in the case of a bipolar transistor) the back emf is effectively clamped to 0.7V because once the diode begins to turn on the property of its pn junction creates a depletion field maintaing the voltage across it at 0.7V. If the diode fails due to wear, temperature etc or exceeding it's break-over voltage the transistor is then exposed to the high back emf and will fail.

Most people forget or do not know/understand the function of the diode (normally referred to as a flywheel or freewheel diode) resulting in repeated failure of replacement transistors.

Hope this is clear.