Research On Reliability Of IGBT Power Module Packaging

Insulated gate bipolar transistors (IGBT) have been the mainstream of power semiconductor devices, and become the key to solve the energy issue through the implementation of power conversion and control. The wide applications of IGBT are closely related to reliability of packaging, which involves multi-disciplines such as thermology, electricity, mechanics and material science. The investigation of reliability and optimal design of IGBT modules is of significance to develop the failure theory of power modules and to promote the module industrialization. Therefore, it is imperative to study the failure mechanisms of IGBT modules, understand the influences of design factors such as packaging materials, technology and structure on the module reliability. This dissertation mainly focuses on the key issues related to the reliability of IGBT power module packaging, including insulated substrate, soldering and heat dissipation. Reliability experiments and analyses are carried out. On this basis, corresponding optimized design schemes are proposed to improve the reliability of IGBT modules.First, failure mode of direct bonded copper (DBC) substrate is explained through thermal cycling test and failure analysis. Failure mechanism of DBC substrate is analyzed theoretically. Criterion for interfacial crack propagation based on stress intensity factor and the maximum tangential stress theory has been established. The process of crack initiation from copper-ceramic interfacial singular point and then propagating into ceramic is investigated through the analytical and numerical methods, respectively. The good correlation of experimental results, theoretical analysis and numerical simulation demonstrates the validity of the criteria for interfacial crack propagation.Next, a design scheme of DBC substrate with step-copper structure is proposed. The statistics of life cycles under thermal cycling indicate that this kind of design can improve the reliability of DBC substrate effectively. Thermo-mechanical coupling model is developed to simulate the thermal cycling process of DBC substrate with step-copper structure. Extremely low cycle fatigue life prediction model is applied to predict its fatigue life. The simulation results agree well with the experimental results. The influences of structural parameters on the stress distribution and lifetime of DBC substrate with copper step are analyzed by the approach of simulation based design of experiment (DoE) in order to optimize the design parameters.Then, thermal fatigue failure mechanism of large area soldering is studied through thermal cycling test and failure analysis. Computational fluid dynamics model and thermo-mechanical coupling model are established. The theoretical analysis and numerical calculation of thermal stress of soldering structure are performed. Strain-based and energy-based life prediction models are applied to predict the fatigue life of solder, and the predictions agree well with experimental results. Solder void evolution during thermal cycling is observed through scanning acoustic microscope. Finite element method is used to study the thermal performance of IGBT module containing solder voids. The functional relationships between junction temperature and void ratio or distributions of voids are established based on the simulation results. Thermo-mechanical transient dynamic model is established to simulate the reflow soldering process. Simulation based orthogonal design is applied to study the influences of multiple soldering structural parameters on the stress distribution in order to obtain the optimal structure design to enhance the reliability of power modules.Finally, microchannel-based technology is applied to fulfill active cooling of high power IGBT modules. Thermo-mechanical performance of IGBT modules embedded with microchannel are investigated by finite element analysis. The temperature distribution during operation, residual stress and warpage caused by reflow soldering, and operating stress and warpage considering the residual stress are studied. The calculated warpages agree well with the coordinate measuring results. The IGBT module embedded with microchannel is nearly flat during operation. The influences of thicknesses of copper baseplate and solder layer on the stress distribution are investigated by parametric sweep method. On this basis, optimal design is proposed to reduce the operating stress and therefore extend the lifetime of the module.