IGBT is Insulated Gate Bipolar Transistor, it’s a composite full-controlled voltage-driven power semiconductor device which is made up by Bipolar Junction Transistor and Metal Oxide Semiconductor. The NTC thermistor produced by EXSENSE Electronics Technology Co., Ltd. plays the role of temperature measurement in IGBT, can measure the temperature of the module substrate accurately. Now, let’s discuss the failure mode and reason of IGBT -- Failure Time and Failure Manifestation.

Failure Time

The failure rate of IGBT will follow the law of Bathtub Curve and show obvious stages. As a result, its failure can be divided into three stages of early failure, stochastic failure and end-of-life failure.

1) Early failure

This situation mainly happens during the stage of production testing or in the early stages of field operation, such as the damage to the NTC thermistor chip during assembly.

2) Stochastic failure

This situation is usually uncontrollable, and stochastic failure rate is high correlated with the operation of IGBT. In the Bathtub Curve, the stochastic failure rate is presented in a lower stable state, which can be represented by FIT (Failure In Time), and the actual meaning is the number of failures within 1 billion hours. (Note: NTC thermistor components with early failure and end-of-life failure must be excluded during the test process)

λ= nf / n*t      FIT =λ*10^9

NF = Quantity of failures (units)

N = Total quantity of devices in operation (pcs)

T = time of observation (in hours)

MTBF (mean Time Between Failures), the mean time of the failure interval, is for the NTC thermistor components which can be repaired.

MTTF (Mean Time To Failure) is the same concept, is for the NTC thermistor components which can’t be repaired.

MTBF and MTTF are the reciprocal to λ.

MTBF=MTTF= 1/λ

3) End-of-life failure

This situation is mainly due to the abrasion of NTC thermistor, its change of power consumption or external environment cause the thermal-mechanical stress, which is inevitable.