Environmental conditions such as high ambient temperature, exposure to mechanical shock, and specific drive cycles require special attention to the mechanical and electrical characteristics of IGBT power modules to ensure that their performance is fully developed and maintained throughout its service life. High reliability. This article discusses the power and thermal cycling of IGBTs.
As many as a dozen Insulated Gate Bipolar Transistors (IGBTs) are commonly used in various industrial applications, IGBT modules are designed to provide optimal cost performance and appropriate reliability for a specific application.
The advent of commercial electric vehicles (EVs) and hybrid electric vehicles (HEVs) has created a new market for IGBT modules. The part of EVs and HEVs that requires the highest reliability of IGBT power modules is the drive train, where the IGBTs are located in the inverter to provide energy to the electric motor of the hybrid system. According to the drive train concept, the inverter can be placed in the trunk of the car, in the gearbox or under the hood close to the internal combustion engine, so the IGBT modules are subjected to severe thermal and mechanical conditions (vibration and shock).
To provide automotive designers with standard industrial IGBT modules with high reliability, IGBT designers must choose materials and design electrical characteristics with special care to achieve similar or even better results.
Thermal Cycling and Thermal Shock Testing
During thermal cycling (TC), the device-under-test (DUT) is alternately exposed to precisely set minimum and maximum temperatures, resulting in a package-to-case temperature difference (ΔTC) of 80K to 100K. The storage time of the DUT at the minimum and maximum temperatures must be sufficient to allow it to reach thermal equilibrium (ie, 2 to 6 minutes). The focus of this test is to examine the fatigue properties of the weld.
Weaknesses in other parts, such as the module's framework, can also be investigated through more rigorous testing. The thermal shock test (TST), also known as the two-box test, is performed under extended ΔTC conditions, such as from -40-C to +150+C, with a typical storage time of 1 hour.
During thermal cycling/thermal shock testing, the DUT is heated externally, while during power cycling (PC) the DUT is actively heated by the load current flowing inside the module. Consequently, both the temperature gradient inside the module and the temperature of the different material layers are much higher than during thermal cycling.
Cooling of the module is achieved by actively shutting down the load current and using external cooling measures. The most typical is the use of water-cooled radiators, but air-cooled systems are also more commonly used. The test device can stop the water flow during the heating stage, and then restart the water flow after entering the cooling stage. Through power cycling, the bond wire connection and the fatigue behavior of the weld can be studied.
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