State detection of bond wires in IGBT modules using eddy current pulsed thermography

Insulated gate bipolar transistor (IGBT) modules are important safety critical components in electrical power systems. Bond wire lift-off, a plastic deformation between wire bond and adjacent layers of a device caused by repeated power/thermal cycles, is the most common failure mechanism in IGBT modules. For the early detection and characterization of such failures, it is important to constantly detect or monitor the health state of IGBT modules, and the state of bond wires in particular. This paper introduces eddy current pulsed thermography (ECPT), a nondestructive evaluation technique, for the state detection and characterization of bond wire lift-off in IGBT modules. After the introduction of the experimental ECPT system, numerical simulation work is reported. The presented simulations are based on the 3-D electromagnetic-thermal coupling finite-element method and analyze transient temperature distribution within the bond wires. This paper illustrates the thermal patterns of bond wires using inductive heating with different wire statuses (lifted-off or well bonded) under two excitation conditions: nonuniform and uniform magnetic field excitations. Experimental results show that uniform excitation of healthy bonding wires, using a Helmholtz coil, provides the same eddy currents on each, while different eddy currents are seen on faulty wires. Both experimental and numerical results show that ECPT can be used for the detection and characterization of bond wires in power semiconductors through the analysis of the transient heating patterns of the wires. The main impact of this paper is that it is the first time electromagnetic induction thermography, so-called ECPT, has been employed on power/electronic devices. Because of its capability of contactless inspection of multiple wires in a single pass, and as such it opens a wide field of investigation in power/electronic devices for failure detection, performance characterization, and health monitoring

Failure Analysis of Thick Wire Bonds

In the last decade, reliability problems have become a critical subject in power modules. Understanding design weakness and failure mechanisms of thick wire bond are two critical steps in managing the risk of wire bond heel crack which is the topic of this thesis. Although this thesis does not target a specific type of power modules, we note that thick wire bond heel crack failures occur in Insulated Gate Bipolar Transistors (IGBTs). In fact, our aim is to understand failure mechanism in 300μm thick wire bonds with different geometries and materials. Since these wires experience harsh environmental conditions and high load transients, the wires undergo repetitive flexural movement which causes heel crack due to fatigue. For the purpose of understanding this failure mechanism, two experimental setups are built and utilized. The first experimental setup loads the wires using constant currents and observes the response using a scanning laser vibrometer to measure the displacement. The second experimental setup applies repetitive prescribed displacement to the first foot of the wire and detects fatigue failure using a Wheatstone bridge. It is realized that wires have different displacement property depending on their geometry and material. Maximum displacements are observed for Al-H11 instead of CuCorAl and PowerCu

Lifetime Estimation of IGBTs in a Grid-connected STATCOM

Lifetime estimation of power semiconductor devices, and IGBT devices in particular, used in the power electronics integrated with power systems has gained technical importance in recent times with increased scope of distributed generation, renewable energy systems and FACTS. Since most of the common failures (wire bond and solder fatigue) are caused by thermo-mechanical stresses, the methodology of lifetime estimation starts with temperature estimation, cycle counting based on rainflow algorithm, and finally degradation calculation based on linear accumulation model. Different number of RC cells for each packaging layer in the module for the thermal model, including the influence of encapsulant is proposed for temperature estimation of IGBTs in power modules. A modified rainflow algorithm with faster execution time and time dependent temperature calculation is introduced for cycle counting. Finally, the lifetime of the IGBT is estimated during STATCOM operation using real-time load profiles for power factor variation. For a power factor variation data for a building, the lifetime is estimated to be about 3 years. Similarly, a month long arc furnace load data is considered to compare the equivalent temperature based calculation to conventional tests. 4% more degradation is observed in the equivalent temperature based calculation than compared with conventional rainflow algorithm. A simulation study on the operation parameter dependence on the stresses in a wire is considered to estimate lifetime from Finite Element Analysis (FEA) in COMSOL. Power cycling tests are conducted on two different modules (600 V, 50 A H-bridge module and a 1200 V, 150 A phase leg module) to validate the lifetime model for four months. The low power module was tested without any protection circuits and hence failed catastrophically. Wire melt-off or fusing failure was dominantly observed, following by dielectric based short circuit failure. The high power module was tested with protection circuits to prevent catastrophic damage for a maximum of 4 months. A maximum of 20% degradation in static characteristics, with decreased on state resistance was observed in the modules. The degradation is attributed to increased junction temperature as the thermal resistance increases owing to solder fatigue

Health Condition Assessment of Multi-Chip IGBT Module with Magnetic Flux Density

To achieve efficient conversion and flexible control of electronic energy, insulated gate bipolar transistor (IGBT) power modules as the dominant power semiconductor devices are increasingly applied in many areas such as electric drives, hybrid electric vehicles, railways, and renewable energy systems. It is known that IGBTs are the most vulnerable components in power converter systems. To achieve high power density and high current capability, several IGBT chips are connected in parallel as a multi-chip IGBT module, which makes the power modules less reliable due to a more complex structure. The lowered reliability of IGBT modules will not only cause safety problems but also increase operation costs due to the failure of IGBT modules. Therefore, the reliability of IGBTs is important for the overall system, especially in high power applications. To improve the reliability of IGBT modules, this thesis proposes a new health state assessment model with a more sensitive precursor parameter for multi-chip IGBT module that allows for condition-based maintenance and replacement prior to complete failure. Accurate health condition monitoring depends on the knowledge of failure mechanism and the selection of highly sensitive failure precursor. IGBT modules normally wear out and fail due to thermal cycling and operating environment. To enhance the understanding of the failure mechanism and the external characteristic performance of multi-chip IGBT modules, an electro-thermal finite element model (FEM) of a multi-chip IGBT module used in wind turbine converter systems was established with considerations for temperature dependence of material property, the thermal coupling effect between components, and the heat transfer process. The electro-thermal FEM accurately performed temperature distribution and the distribution electrical characteristic parameters during chip solder degradation. This study found an increased junction temperature, large change of temperature distribution, and more serious imbalanced current sharing during a single chip solder aging, thereby accelerating the aging of the whole IGBT module. According to the change of thermal and electrical parameters with chip solder fatigue, the sensitivity of fatigue sensitive parameters (FSPs) was analyzed. The collector current of the aging chip showed the highest sensitivity with the chip solder degradation compared with the junction temperature, case temperature, and collector-emitter voltage. However, the current distribution of internal components remains inaccessible through direct measurements or visual inspection due to the package. As the relationship between the current and magnetic field has been studied and gradually applied in sensor technologies, magnetic flux density was proposed instead of collector current as a new precursor for health condition monitoring. Magnetic flux density distribution was extracted by an electro-thermal-magnetic FEM of the multi-chip IGBT module based on electromagnetic theory. Simulation results showed that magnetic flux density had even higher sensitivity than collector current with chip solder degradation. In addition, the magnetic flux density was only related with the current and was not influenced by temperature, which suggested good selectivity. Therefore, the magnetic flux density was selected as the precursor due to its better sensitivity, selectivity, and generality. Finally, a health state assessment model based on backpropagation neural network (BPNN) was established according to the selected precursor. To localize and evaluate chip solder degradation, the health state of the IGBT module was determined by the magnetic flux density for each chip and the corresponding operating conduction current. BPNN featured good self-learning, self-adapting, robustness and generalization ability to deal with the nonlinear relationship between the four inputs and health state. Experimental results showed that the proposed model was accurate and effective. The health status of the IGBT modules was effectively recognized with an overall recognition rate of 99.8%. Therefore, the health state assessment model built in this thesis can accurately evaluate current health state of the IGBT module and support condition-based maintenance of the IGBT module

Design of a signal chain for generation of a high power and high frequency signals used for determination of bond wire lift-offs based on magnetic field

Inverters are a power electronic module widely used nowadays in fields like renewable energy or automotive industry. They consist of a set of IGBTs, which are very fragile devices. One of the main failures of IGBTs is the bond wire fatigue, resulting in bond wire lift-off. For the early detection of these failures, the state of bond wires must be monitored. In this study, the lift-off failure was recreated by means of a set of experiments. The bond wires were cut manually for different conditions and the magnetic field that the remaining wires emitted was measured with GMR sensors. Several measurements were performed varying different parameters, such as the length of the wires, sensor position or type of signal. After the measurements, the obtained results were analyzed. The purpose of this analysis was to find a pattern able to recognize whether lift-offs happened and how the wire parameters influence the sensor outputs. The results can be a contribution for future research to generate a predictor that allows to detect in real time whether and where any lift-offs are generated in power electronic modules

In-situ Health Monitoring Applied to High-Voltage IGBT Power Modules

PhD ThesisThis thesis addresses an important issue of identifying insulated gate bi-polar transistor (IGBT) chip failures in multichip IGBT power modules. IGBT power modules are the dominant semiconductor devices of choice in high-voltage (HV) high-power converter applications which include domestic, commercial, automotive, railway, marine, aerospace and industrial applications. Commonly available HV IGBT power modules in the market are rated at 3.3 kV, 4.5 kV and 6.5 kV. These HV IGBT modules comprise several IGBT chips connected in parallel to achieve high-current capability; hence they are also known as multichip IGBT power modules. IGBT power modules are not flawless. The increased complexity of IGBT power module construction and inhomogeneous semiconductor chips make HV power modules less reliable. IGBT chips and electrical and mechanical interface material within the modules wear out and fail due to thermal cycling, operating environment or mishandling. IGBT failures while in application have repercussions on safety and failure costs. Thus the reliability of IGBTs while in their application is crucial especially in HV applications which comprise critical and large loads. To improve the reliability, an in-situ (online) health monitoring interface for HV IGBT power modules is proposed in this thesis. Two distinct advantages of in-situ IGBT health monitoring are that it allows IGBT module replacement prior to complete failure thus reducing safety and reliability risks. The second advantage is that the interval time for IGBT maintenance work can be tailored towards the real degradation rather an obligatory fixed time interval thus reducing maintenance costs. In large power modules, it is common to have IGBT chips as well as anti-parallel diode chips within the power module. This research focusses only on the health monitoring of the IGBT chips and not the diode chips. The main reason is that IGBT chips experience higher thermal stresses compared to diodes hence IGBT chips are more susceptible to failures compared to diode chips. In practice, IGBT chip failures are accompanied by a change in junction temperature. Thus this thesis proposes the use of temperature- sensitive electrical parameters (TSEPs) for in-situ health monitoring of IGBT power modules. Following a comparison of twelve traditional online TSEPs from literature and five new TSEPs proposed in this thesis, this thesis employs a novel TSEP, gate-emitter prethreshold voltage (VGE(pre-th)) as a health-sensitive parameter (HSP) for chip failure detection in multichip HV IGBT power modules. A VGE(pre-th) online chip loss monitoring circuit has been successfully implemented on a commercially available IGBT gate driver. VGE(pre-th) is measured at a fixed pre-determined instant of the gateemitter voltage (VGE) between the VGE zero-crossing (VGE(0)) and threshold voltage (VGE(th)) during IGBT turn-on. VGE(pre-th) requires low hardware with only a voltage sensor and a counter. Since it is based on the low-voltage (LV) gate side rather than the HV collector side of IGBT, VGE(pre-th) does not require HV isolation or HV insulation. Simulation and experimentation of 16-chip 3.3kV 800A DIM800NSM33-F IGBT power modules from Dynex Semiconductor Limited (Ltd) have shown that VGE(pre-th) has a good accuracy and repeatability; a linear sensitivity of 500 mV/chip loss with IGBT chip failures; a linear virtual junction temperature (Tvj) sensitivity of -2.2 mV/°C and tracks the highest chip temperature. It has thus been concluded that VGE(pre-th) can be used for both Tvj and IGBT chip failure monitoring in HV IGBT power modules. VGE(pre-th) can be tested during normal IGBT turn-on operation or during the off-state of the IGBT. In both cases the same information about temperature and loss of chip number can be detected which makes VGE(pre-th) more versatile than any other TSEP or HSP.Engineering and Physical Sciences Research Council (EPSRC), Newcastle University