Czerny, Bernhard

2012-09, Czerny, Bernhard, Lederer, Martin, Nagl, Bernhard, Trnka, A., Khatibi, Golta, Thoben, Markus

The lifetime of IGBT (Insulated Gate Bipolar Transistor) modules is limited by thermo-mechanical fatigue. Thereby bonding wires represent the critical links where damage initiation is observed. For the first time Laser Doppler Vibrometer measurements and thermal imaging were employed to determine the temperature-dependent deformations of bond wires at different frequencies under operation conditions. This should be considered as an important step to facilitate more precise life-time predictions of power modules in long term usage.

2012-04-16, Czerny, Bernhard, Nagl, Bernhard, Lederer, Martin, Trnka, A., Khatibi, Golta, Thoben, Markus

The subject of this investigation was determination of thermo-mechanically induced displacement of the components inside a power module under operation conditions. It is well known that lifetime of insulated gate bipolar transistor (IGBT) modules is limited by thermo-mechanical fatigue. Wire bonded interconnects inside the IGBTs count as critical sites where crack initiation and failure is observed. In this study the temperature dependent periodic deformation of wire-bonds under operating conditions was determined by using a laser Doppler vibrometer (LDV) and thermal imaging camera. Furthermore finite element analyses (FEA) were conducted to obtain the strain values needed for lifetime assessments.

2012-09-17, Czerny, Bernhard, Nagl, Bernhard, Lederer, Martin, Khatibi, Golta, Thoben, Markus, Nicolics, Johann

This paper discusses different application relevant electrical loading cases of an IGBT module of a power inverter. Thereby, different operation conditions such as pulse frequencies, inverter output currents and output frequencies, as well as two different operation modes are discussed. Each load case investigation is conducted by electrical, thermal, and in situ vibration measurements. Moreover, on the base of finite element analyses a deeper insight is gained into reliability relevant thermo-mechanical behavior. For this purpose an IGBT module is operated at a load of 30% to 80% of its nominal value in order to cause representative thermo-mechanical displacements of dies and bond wires. By applying an inverter output frequency in a range of 1 to 280 Hz a temperature ripple of up to 40 K on the dies and a vertical displacement of up to 9 μm on a bond wire is observed. These results are important to improve life-time-predictions.

2013-04-14, Czerny, Bernhard, Khatibi, Golta, Thoben, Markus, Paul, Indrajit

The subject of this study was to investigate the effect of different geometrical loop shapes on the reliability of 400 μm thick Al bond wires in IGBT modules by means of experimental and analytical methods. The experimental fatigue tests have been realized by linear cyclic displacements of 5-45 μm of the contact plates at 200 Hz and 20 kHz. Life time curves were obtained for bond wire connections with different loop heights, distances and angles with the main failure mechanism being wire bond heel cracking. Furthermore an analytical model was developed to calculate the effect of variation of geometrical shape parameters on the stress at different locations of the bond wire. This model can be used to make a preliminary geometry selection of the bond wire and to predict the force or stress at critical sites of the wire bond during stress tests. This model was validated with finite element analysis.

2013-05-26, Czerny, Bernhard, Thoben, Markus, Khatibi, Golta, Lederer, Martin, Nagl, Bernhard, Nicolics, Johann

This study comprises the electrical analysis of an experimental investigation on thermo-mechanical vibration measurements on an IGBT inverter structure under operating conditions and shows a new way how to experience reliability relevant phenomena. In order to perform transient temperature measurements with IR thermography and optical vibration measurements one sub-system of the inverter module was extracted and operated at equivalent conditions. Necessary circuit modifications including parasitic impedances and their most important influences are discussed. The investigation revealed a strong dependence of the thermo-mechanical bonding wire vibrations on the inverter output frequency. At 1 Hz an amplitude of more than 4 μm was measured at the loop peak of a short bonding wire.

2013-04-14, Czerny, Bernhard, Lederer, Martin, Nagl, Bernhard, Trnka, A., Khatibi, Golta, Thoben, Markus

Fatigue failure of wire-bonds is one of the key factors limiting the lifetime of power electronic devices. In IGBT (insulated gate bipolar transistor) modules, wire-bonds are exposed to repeated temperature changes leading to thermo-mechanical stresses in the constituent materials. Due to the geometry, stress concentrations arise at the interfaces of aluminum wires and silicon chips. In the framework of classical continuum mechanics, these stress concentrations show the characteristics of stress singularities. Nevertheless, IGBT modules reach lifetimes of about 30 years under service conditions. Therefore, it seems that classical continuum mechanics exaggerates the stress concentrations occurring at the material transitions. Hence, it is the subject of the present investigation to calculate more realistic stress distributions using a novel strain gradient theory.

2013-09, Czerny, Bernhard, Khatibi, Golta, Paul, Indrajit, Thoben, Markus

The reliability of power electronic devices is mainly limited due to thermo-mechanical fatigue of the internal bond wire interconnections. The bond wire shape is already defined at the design stage of the device. Thus preliminary lifetime assessments become more and more important in order to satisfy the high quality demands and the short time to market of the devices. In this study a fast experimental test setup is used in order to determine the lifetime of a large number of wire bond shapes. Furthermore an analytical model is applied to calculate optimized wire bond shapes for a given set of parameters. The results of this investigation should help to optimize the shape parameters at an early stage of development using the presented analytical model in combination with the fatigue tests. © 2013 Elsevier Ltd. All rights reserved.