With the development of offshore wind power, photovoltaics, and electric vehicles (practical outdoor working conditions), the influence of humidity on power electronics’ operation reliability has been emphasized. However, the influence mechanism of humidity on packaging reliability is still uncertain. Some literature found that the thermal impedance may be influenced by humidity and further affects the power cycling lifetime, but this phenomenon needs further verification and mechanism analysis. In this paper, the experiment is designed, and the influence mechanism of humidity on the thermal resistance of power electronics is proposed. The results indicate that moisture can invade DUT and cause transient thermal impedance Zth variation in two aspects. One corresponds to the chip solder layer of DUTs, and the other corresponds to the case surface contact resistance of the DUTs. Furthermore, it is revealed that the thermal capacity, density, and conductivity are influenced by humidity, resulting in thermal impedance variation. Firstly, the information on the DUTs, the basic principle of transient thermal impedance Zth measurements, the test platform, and the process are introduced. This paper uses three kinds of DUTs (TO-247 of SiC MOSFET, EasyPack of SiC MOSFET, and 34 mm Module of Si IGBT). EasyPack modules have no substrate, and 34 mm modules have a substrate. TO-247 devices and EasyPack modules only have a chip solder layer, while 34 mm modules also have a system solder layer, which can characterize the moisture absorption capacity at different layers. For the junction-heatsink transient thermal impedance (Zthjs) measurement of Si IGBT devices, the gate is fully turned on during the test. In contrast, for SiC MOSFET devices, the gate must be turned on to conduct the load current and turned off to ensure that the measurement current source entirely passes through the reverse body diode and the junction temperature is measured with VSD(T). DUTs are placed in a constant temperature humidity chamber, and the gate driver circuit board, current source, and cooling system are placed outside. The experiment is performed at 85°C/85%RH storage for 722 hours with ten times Zthjs measurements (the time interval between two Zthjs measurements is 72 h), and the change between two adjacent Zthjs is observed to evaluate the influence of humidity on thermal resistance. Then, the experimental results are discussed. Moisture can diffuse from the environment into the DUT, further leading to changes in the DUT’s Zthjs. All TO-247 devices, the Easy-4 module, and all 34 mm modules indicate a change in chip solder layer thermal resistance. Furthermore, the influence of humidity on the DUT’s Zthjs comes from the thermal resistance of the chip solder inside the device and the contact thermal resistance of the device case surface. The copper substrate is corroded when long-time submerged in a high-humidity environment. Moreover, the influence mechanism is proposed. When moisture is 1%0 of the solder content, the solder layer’s thermal capacity, density, and conductivity change to 101.9%, 100.013 5%, and 91.4% of the original values, respectively. With the increase of the chip solder layer’s thermal capacity, the solder layer’s thermal resistance in Zthjs measurements decreases. Finally, the conclusion can be drawn: (1) Moisture can invade power electronics, resulting in thermal impedance variation for three kinds of DUTs. (2) The humidity can influence the chip solder layer when thermal resistance is changed by humidity and case surface when corrosion occurs in copper material. (3) The thermal characteristic between moisture and chip solder layer is different. The moisture invasion leads to an increase in thermal capacity and density, and a decrease in thermal conductivity, further influencing the power electronics’ operation lifetime. It is the reason for Rth change at the chip solder layer. © 2024 China Machine Press. All rights reserved.
