Bernhard Czerny

Thermo-mechanical analysis of bonding wires in IGBT modules under operating conditions

Abstract 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.

Experimental and analytical study of geometry effects on the fatigue life of Al bond wire interconnects

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.

Electro-thermal analysis of in situ vibration measurements on IGBT modules under operation conditions

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.

A novel approach for evaluation of material interfaces in electronics

The rapid technological advancements and market demands in electronic sector requires application of highly accelerated, still practice relevant reliability assessment methods. At present, accelerated power and temperature cycling tests count as the state of the art for qualification of the devices. However due to physical characteristics of the devices, there are limitations to accelerated thermal and power cycling tests. Further acceleration by exceeding a critical temperature or time reduction may result in occurrence of failure mechanisms other than those encountered in real application or suppressing these failures. An alternative approach for further acceleration of the testing procedures is based on the application of isothermal mechanical fatigue testing at high frequencies (AMT). The principle idea of this approach is replacement of thermally induced strains by means of equivalent mechanical strains. Based on a physics of failure approach, the relevant failure modes in the material interfaces are induced enabling detection of weak sites of the devices in a very short duration of time. In addition of time saving factor a further advantage of mechanical fatigue testing is the possibility of decoupling of thermal, mechanical and environmental stress factors for a more effective investigation and diagnosis. This paper presents an overview of our recent reliability studies on different types of electronic components by using the proposed methodology with the aim to give an insights into the advantages and some restrictions of AMT for qualification of electronic devices.

A New Approach for Evaluation of Fatigue Life of Al Wire Bonds in Power Electronics

Ultrasonically bonded Al wire bonds on Al metallization pads are widely used in power semiconductors. The required long time reliability of the devices is highly dependent on the interfacial quality of Al wire and the bond pad. Reliability of wire bonds is commonly assessed by thermal and power cycling tests. Accelerated mechanical fatigue testing can be used as an alternative to these time consuming procedures. In the present study, lifetime of thick Al wedge bonds on Si substrates was investigated using a novel mechanical fatigue testing technique operating at high frequencies and elevated temperatures. The influence of microstructure, testing temperature and frequency on lifetime of Al wire bonds was investigated. Finite element analysis was applied to calculate the stress distribution at the interfacial region and to establish life time prediction curves. The results of mechanical isothermal fatigue curves were compared and correlated with thermal cycling data of Al wire bonds.

Fatigue testing method for fine bond wires in an LQFP package

Abstract A mechanical testing setup was developed to study the fatigue response of fine thermo-sonic wire bond connection in low profile quad flat packages (LQFP). The testing set-up was designed to induce pre-defined multi-axial stresses in the wire bond loops of non-encapsulated packages in order to mimic their deformation behavior during the thermo-mechanical loading. Lifetime curves were obtained up to 1.0E7 loading cycles with fatigue failure occurring in the heat affected zone of the ball bond. The experimental fatigue data in combination with extended FEA provided the basis for a Coffin Manson lifetime model. The proposed fatigue testing procedure can be applied as a highly efficient method for evaluation of various wire bonded packages by using a limited number of test samples and simultaneous testing of several wire bonds.

Wire bond degradation under thermo- and pure mechanical loading

Abstract This paper presents a fundamental study on degradation of heavy Al bond wires typically used in high power modules. Customized samples are designed to only consist of Al bond wires on standard Si diodes. These samples are subjected to pure mechanical and passive thermal cycling to investigate the bond degradation behavior on a simple system as well as compare these two test methods. Although an appreciable difference in fracture behavior is observed between these two methods, both provide correlation between the number of cycles and degree of degradation, especially in the case of the passive thermal test. To enable investigation of degradation rate a large number of bond interfaces is analyzed and they are found to follow conventional accepted fracture laws like Paris-Erdogan. With additional work this could enable the possibility of obtaining empirical parameters to be used in actual physics based lifetime laws.

Experimental investigation of transient electrical, thermal and mechanical behavior of IGBT inverter modules during operation

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.

A fast test technique for life time estimation of ultrasonically welded Cu-Cu interconnects

Abstract In this research the quality of the interconnects of the ultrasonically welded Cu terminals to the Cu substrate in the IGBT-module has been investigated. An ultrasonic resonance fatigue system in combination with a laser Doppler vibrometer and a special specimen design was used for shear fatigue testing of these large ultrasonic Cu–Cu welds (about 0.5 cm2). Fatigue life curves up to 109 loading cycles were obtained in a very short period of time. Using this technique it was possible to evaluate the fatigue strength of these interconnects for the first time. The microstructural features of the interconnects were characterized and their crack growth behaviour was studied. Fracture analysis of the fatigued specimen shows that failure occur due to the propagation of the crack beneath the welding interface into the copper substrate. Additionally performed finite element simulations offer an insight into the stress and strain concentrations during the mechanical fatigue tests. As this method is not restricted to the welding geometry, material joints with larger interconnects can be tested likewise. Thus this new technique can be used as a practical and valid fatigue testing method for evaluation of various interconnects.