Microstructure coarsening of high-temperature Sn-Pb solder joint in IGBT modules of high-speed trains

The insulated gate bipolar transistor (IGBT) is one of the important electronic devices to control the complex circuit system of electric high-speed trains. The reliable function of IGBT is the guarantee for the efficient operation of vehicles. The Sn25Pb solder joint between the chip and aluminum-clad ceramic plate plays a role in support and heat dissipation in packaging structure. On the other hand, it is a weak point in terms of reliability. Failures often occur in this structure when a vehicle undergoes several years of operation and power cycling tests. Thus, it is imperative to understand the failure mechanism of solder joints in IGBT modules.In this study, several groups of IGBT modules underwent different service times, and conditions were selected firstly. These conditions include newly made, 5 as well as 10 years of operational times. Secondly, we measured the area of the coarsened Pb phase in these samples by a scanning electron microscope (SEM) to quantify the coarsening of the solder joint during service. Also, the thickness change of the intermetallic compound layer (IMC) was measured. In addition, the cracks were located, and the failure mechanism was determined. Finally, the hardness of Sn25Pb solder under different coarsening levels was obtained by Vickers hardness indentation tests. The correlation between solder’s hardness and its lifespan was proposed. During investigation, SEM and ImageJ software were used to examine the cross-sectional images quantitively.