Christian Zardini

Strategy for designing accelerated aging tests to evaluate IGBT power modules lifetime in real operation mode

Accelerated aging tests are widely used in electronics industry to validate the technological choices of packaging, especially when significant lifetime is required. The thermal coefficient of expansion mismatch between the soldered materials results in crack propagation in the solder joints, which irreversibly increases the thermal resistance of the assembly and progressively leads to the failure of the module. Accelerated testing is very expensive and can last a long time, so that such experiments must be optimized. In addition, thermomechanical behavior of the assembly under realistic operating conditions for long times can be studied in shorter times thanks to finite element simulations. Non-linear finite element simulations have been carried out to make a correlation between accelerated aging tests and real operation. Stress-strain history under representative thermal loading has been determined in both cases. The energy dissipated in solder joints has been calculated and has been used as an estimator of the damage in the solder joint. Finally, thermal fatigue experiments with representative samples have allowed validating the previous results. This study is a step toward the understanding of the correlation between accelerated testing and actual operating conditions.

A systematic hard- and soft-switching performances evaluation of 1200 V punchthrough IGBT structures

The performances of various 1200 V insulated gate bipolar transistor (IGBT) structures for hard- and soft-switching (both the zero-voltage and the zero-current switching) at high temperature and under various test conditions are investigated in detail. A comparison is made between a conventional planar punchthrough IGBT (P-IGBT) using global lifetime control, a trench punchthrough IGBT (T-IGBT) using local lifetime control and a enhanced planar punchthrough IGBT (N-IGBT) also using the local lifetime control process. A simple and effective specific test circuit allows hard- and soft-switching operating modes, and provides also an independent control on the test parameters (load current, clamping voltage, gate resistance, temperature, dead times). It is highlighted that the local lifetime control is very efficient at high temperature reducing especially the turn-off losses. Furthermore, the T-IGBT exhibits very good performances during hard- and soft-switching turn-off, but this is not the case during hard-switching and zero-voltage switching turn-on. Due to its high input capacitance, the hard-switching turn-on losses are relatively high. Regarding the N-IGBT, it presents good performances under hard-switching and also zero-voltage switching. However, like the T-IGBT, its structure has a strong inductance behavior during zero-voltage turn-on increasing the conductivity modulation lag and also the voltage spike leading to high losses. All these data allow to provide useful information for the devices modeling, design, and optimization.

Study of ultracapacitors dynamic behaviour using impedance frequency analysis on a specific test bench

This paper presents a study on the ultracapacitors dynamic behaviour. Frequency analysis is chosen as the tool to perform this investigation. First, the impedance spectroscopy principles are presented. Then the experimental results, obtained by applying impedance spectroscopy with high current levels, are presented and discussed. Ultracapacitors show a particular behaviour which leads to it specific model composed by a small serial inductance associated with a four RC-branches network.

Switching performances comparison of 1200 V punch-through and nonpunch-through IGBTs under hard-switching at high temperature

Switching performances comparison of 1200 V punch-through (PT) and nonpunch-through (NPT) IGBTs under hard-switching at high temperature has been studied in detail with the aid of extensive measurements at various temperatures, gate resistances and clamping voltages. The test circuit allows the characterisation of the switching losses of the IGBTs under hard-switching independently of those due to the freewheeling diode. We show that these losses are quite similar to each other during turn-on, but more important for PT-IGBT during turn-off. Moreover, NPT-IGBT is less sensitive to temperature and clamping voltage variations than PT-IGBT.

Reliability of Lead-Free BGA Assembly: Correlation Between Accelerated Ageing Tests and FE Simulations

In 2006, lead was banished in electrical and electronic equipment in Europe, but the reliability of lead-free technology is not yet well known. This paper describes a methodology using experiments and FE simulations that allow us to assess acceleration coefficients (ACs) and to predict assembly lifetimes. Two accelerated ageing tests have been conducted on lead-free ball grid array (BGA) assemblies. During the ageing test, cycles-to-failure have been accurately determined by an event detector from AnaTech. The 3-D nonlinear finite-element model of the BGA assembly has been designed and built using ANSYS software. Thermomechanical simulations have been carried out on this model to compute the strain energy density (SED) that dissipated in the solder joints during the thermal cycles. Then, the correlation between the experimentations and the simulations has allowed us to assess the ACs of two different kinds of thermal cycles and, thus, the cycles-to-failure for another test.

Ultracapacitors modeling improvement using an experimental characterization based on step and frequency responses

This paper deals with ultracapacitors modeling that has been improved for high power source purpose such as in hybrid and electric vehicles applications. An experimental method based on the combination of step and frequency response has been implemented and used in a dedicated test bench. First, a critical overview of available ultracapacitors models is made and a new approach is proposed. Then, after constant current and impedance spectroscopy tests, the experimental results are presented and discussed. Finally, the exploitation of the tests results leads to the identification of a proposed model parameters. This electric circuit is validated by comparing simulation to experimental curves.

Thermal fatigue resistance evaluation of solder joints in IGBT power modules for traction applications

Several ageing test campaigns have been carried out to evaluate thermal fatigue resistance of solder alloys used in IGBT power modules. The solder joint between the ceramic substrate and the copper baseplate of this stack packaging has been the focus of these experiments. Indeed, this interconnection layer is highly subject to shear strains and stresses due to the mismatched expansion on either side of the solder joint during operation. The impact of the substrate metallization and of the cooling rate has been evaluated by measuring crack growth rate on cross sections of cycled solder joints. Two solder alloy compositions have been selected: a lead-free one and a lead-bearing one. Scanning electronic microscopy analyses have allowed a study of cracking at the microstructural level. Moreover, additional analyses based on scanning acoustic microscopy have led to a ranking of the batches under test. Best results in terms of thermal fatigue resistance have been obtained with the lead-bearing solder alloy. Furthermore, it has been shown that the use of bare copper substrates, and above, all fast cooling rate after the solder reflow, delay solder joint failure.

Experimental study of sources hybridization electromechanical storage system integration into an electric vehicle structure

This paper deals with the design and the integration of an electromechanical storage system into an electric vehicle power train. The main goal is to prove the interest of sources hybridization for heavy duty vehicles with high discontinuous mission profile as garbage collection. This behavior is characterized by a high peak-to-continuous battery power ratio. To optimize vehicle performances, the battery must be sized for the continuous power consumption or recuperation. In this case, the battery can be considered as an energy source. Peak power during acceleration or braking is produced or received respectively by the electromechanical storage system which acts as a power source. This principle has been validated thanks to simulations in Matlab/Simulink environment. Then, to prove these results, we have implemented the power source on a dedicated test bench which allows to recreate the dynamic vehicle behavior on a given mission profile. Experimental results are shown and discussed.

A study of 600V punch-through IGBT dynamics under unclamped inductive switching

This paper deals with the 600 V punch-through insulated gate bipolar transistor (IGBT) dynamics under nondestructive unclamped inductive switching (UIS) conditions. These extremely high stress conditions allow to determine the ruggedness of the device. This can be done by evaluating the maximum of avalanche energy which can be handled by the device submitted to the discharge of the unclamped inductive load. This analysis is performed by using a physically-based two-dimensional semiconductor device simulator associated with a circuit simulator which allows to apply boundary conditions. The simulation results, in good agreement with experiments, allow to investigate the dynamics of the IGBT. These results show that the static breakdown voltage value is not reached due to an excess of positive charges in the drift region. Furthermore, the dissipated energy is maximum under the Pbase region of the device, where the power density is the highest, as well as the temperature value.

Thermal transient characterization of electronic assemblies by infrared thermography and Delta Vbemeter methods

The severe operating environment of electronic systems, means that the design of electronic boards requires continual investigations, involving not only electronics but also thermal and thermomechanical studies. We present here two complementary experimental methods to characterize the thermal transient behaviour of electronic assemblies and we compare the results obtained. Then, we conclude by presenting the fields of applications for such test equipment.