Transient Thermal Impedance Measurement Of Power Electronics Attached By Pressureless Sintered Nanosilver Paste

As a new kind of lead-free die attachment material, nanosilver paste has achieved attention because it has great mechanical, electrical and thermal properties, which can meet the requirements of high temperature and high power density packaging.Scholars have conducted much study on the mechanical properties of the sintered joints, whereas the thermal properties are paid less attention to. This study proposed a transient thermal impedance(Zth) measurement method using gate-emitter voltage(VGE) of Insulated Gate Bipolar Transistor(IGBT) as the temperature sensitive parameter(TSP). The thermal calibration coefficient of VGE with respect to junction temperature(TJ), i.e. the K-factor, is about 10.3 mV/oC. By controlling the effective heating time, the Zth of each component within the packaged device could be measured accurately with error less than 0.87%. The factors affecting Zth measurement, such as the environment temperature, heating power dissipation, heating duty-cycle, and heating time were investigated. It was found that the Zth increased with the environment temperature, heating voltage, heating duty-cycle, and heating time, but almost not sensitive to the heating current.In addition, it usually needs assisted hot-pressure to sinter the nanosilver paste at low-temperature for interconnecting high power devices, which is prone to bring about failure of silicon-based die. Besides, the hot-pressure equipment is complex and costly, which is not conducive to the application of nanosilver paste. Consequently, in this study, a pressureless low-temperature sintering profile of nanosilver paste for attaching power chips was investigated. By analysis of the micromorphology, thermal performance and mechanical properties of the sintered joints, the effects of sintering temperature and dwelling time on the properties of the sintered joints were studied. It was found that the Zth of the joints sintered at 250oC for 10 min were nearly stable. The average shear strength, thermal conductivity, transient thermal impedance, and porosity of the sintered joints at 250oC for 10 min is 25 MPa, 300 W/m·K, 0.05 oC/W and 20%, respectively, which shows good thermo-mechanical performance.Finally, the ansys simulation shows that the voids within the joint have great effects on its thermo-mechnical prrformance. Based on the simulation, the relationship of the Zth and shear strength(?) was fitted with the equation ?=6.69e(0.043/Zth). With the equation, the ? could be deduced by measuring its Zth nondestructively. Evaluating the mechanical properties of power electronics using thermal properties testing nondestructively has significance in characterizing the die-attach quality in industrial applications.