From <Failure Mode and Reason of IGBT -- The Role of NTC Thermistor-1>, we analyzed the failure mode and reason of IGBT from the Failure Time. Today, we analyze the failure mode and reason of IGBT from Failure Manifestation.
From the point of failure manifestation, the failure of IGBT can be divided into three forms of electrical over load, mechanical over stress and environmental influence.
1) Electrical Over Stress
This situation means that during testing, production, or application, overpressure and overcurrent cause IGBT to exceed the safe operating range and result in failure. Overvoltage can be divided into gate pole/emitter overpressure, overvoltage between collector/emitter, etc. Overcurrent can be divided into overcurrent caused by short circuit, continuous overcurrent of load current, instantaneous current is too large during switching process, etc. In addition to the direct influence of voltage and current, the thermal Resistance RTH (J-A) of the system, the loss of NTC thermistor, the wrong improper maintenance of the heat dissipation system in practical applications, etc., may lead the accumulation of heat to fail.
In different cases of electrical overload, the failure state shown under the same test conditions are different. In the case of the same failure state, the corresponding root causes are different in thousands of ways. Therefore, when the electrical overload stress fails, it’s necessary to combine the failure state, pay close attention to the statistical failure time and location statistics of the failure rate, and verify whether there are any changes leading to the failure occurred from five aspects of people, machine, material, method and step.
2) Mechanical Over Stress
The most common case of mechanical over stress is the insulation failure of the module after device control block ruptures.
The ceramics in the device control block are Alumina or Aluminum Nitride generally, and improper operation during installation and transportation may lead to ceramics cracked.
3) Environmental Influence
Environmental influence can be divided into moisture and condensate water, dust, chemical corrosion and so on.
Take moisture and condensate water as an example, the difference in water content within the seal has a balanced process between different sealing materials. For instance, the external relative humidity is larger, after a certain period of time, the higher humidity will also pass through the silicone, to the surface of NTC thermistor chip, its surface storage of water ions will bring about changes in the electromagnetic field.