Study on the methods to measure the junction-to-case thermal resistance of IGBT modules and press pack IGBTs

The accurate measurement of the junction-to-case thermal resistance of Insulated Gate Bipolar Transistor (IGBT) devices is notably important for manufacturers to optimize the internal structure of packaging in order to improve its reliability and for users to take full advantage of the devices. The existing differences between IGBT modules and press pack IGBTs (PP IGBTs) not only in their packaging styles, but also in their working conditions may lead to some differences in the methods to measure their junction-to-case thermal resistance. In this paper, the junction-to-case thermal resistances of both IGBT modules and PP IGBTs have been measured using the traditional thermocouple method (steady-state method) and transient dual interface method (transient method). The applicability of these two methods for the measurement of junction-to-case thermal resistance of IGBT modules and PP IGBTs is summarized based on the experimental results. The steady-state method is suitable for the measurement of junction-to-case thermal resistance of IGBT modules, but not for PP IGBTs, because of the thermocouple inserted to measure the case temperature. The transient method is appropriate for the measurement of junction-to-case thermal resistance of not only IGBT modules, but also PP IGBTs, as a thermocouple is not required to measure the case temperature.     

Optimization of the thermal contact resistance within press pack IGBTs

The research of thermal contact resistance between multi-layers within press pack IGBTs (PP IGBTs) is significant for optimizing the PP IGBTs' thermal resistance to improve reliability, as the thermal contact resistance accounts for approximately 50% of the total thermal resistance of PP IGBTs. In this paper, thermal contact resistance between multi-layers is analysed via a finite element model (FEM) of a single fast recovery diode (FRD) submodule. Most importantly, the influence of temperature and clamping force on the thermal contact resistance is also discussed, and findings are verified by submodule thermal resistance experiments. Based on the FEM and experimental results, nanosilver sintering technology is proposed to fill the gap between the contact interfaces to reduce thermal contact resistance. The fabrication of a sintered single FRD submodule is also investigated in this paper, and the results of the sintered sample indicate that the thermal resistance is reduced by approximately 18.8% compared to a direct contact sample.     

The robustness of series-connected high power IGBT modules

The behaviour in terms of robustness of series-connected high power IGBT modules is presented, arranged in a topology which ensures voltage balance on IGBT’s and diodes by means of a simple auxiliary circuit applied directly on the high power devices, which are used in hard switching mode. Analyses in terms of IGBT and diode SOA (safe operating area), collector to emitter voltage gradient and short circuit condition are reported as well as an extended experimental characterisation. Both analyses confirm superior switching rating and system reliability, by using two series-connected IGBT in substitution of a single module, same current and double voltage rated. Moreover, thanks the auxiliary circuit presence, the robustness of total system is maintained also in extreme operating conditions.     

Substrate-to-base solder joint reliability in high power IGBT modules

Acoustic microscope imaging proved to be an excellent tool to detect and quantify solder fatigue of the substrate to base interface of high power IGBT modules. This technique was used to establish the dependence of the thermal cycling capability on the temperature swing of the module base for a A1N/Cu system. Results from temperature cycling tests were combined with results from power cycling tests to predict the solder joint reliability over a wide range of temperature excursions.     

Instable mechanisms during unclamped operation of high power IGBT modules

The behaviour in terms of robustness during unclamped operations of power IGBT modules is presented. The experimental characterization is aimed to identify the main instable phenomena during unclamped turn-off in power IGBT modules. Several devices of different generations, current and voltage ratings have been analyzed. Thanks to a non-destructive experimental set-up, it is possible to observe instable phenomena without causing the damage of the device under test. In this paper, it is shown that the destructive conditions during unclamped operations are preceded by precursors on the gate side which indicate instable phenomena taking place inside the device. The dependence of the destructive phenomenon on the driver conditions are widely and exhaustively analyzed.     

Degradation mapping in high power IGBT modules using four-point probing

An electrical four-point probing approach is used to estimate local degradation in high power insulated gate bipolar transistor modules subjected to power cycling. By measuring electrical parameters of selected units and components the possibility of mapping the degradation is demonstrated. The development of failures is put in accordance with physical phenomena and materials fatigue. These results are directly usable for reliability purposes with a focus on geometry optimization and enhanced lifetime prediction methods.     

Double-sided cooling for high power IGBT modules using flip chiptechnology

A new technique for the packaging of IGBT modules has been developed. The components are sandwiched between two direct bond copper (DBC) substrates with aluminum nitride. Wire bonds are replaced with flip chip solder bumps, which allows cooling of components on both sides. Microchannel heat sinks are directly integrated in the package to decrease the thermal resistance of the module. Thus, a very compact module with high thermal performance is obtained. A prototype with two insulated gate bipolar transistors (IGBTs) and four diodes associated in parallel was realized and tested. In this paper, the innovative packaging technique is described, and results of thermal tests are presented     

Boundary element analysis of thermal fatigue effects on high power IGBT modules

The technology of high power IGBT modules has been significantly improved these last years against thermal fatigue. The most frequently observed failure modes, due to thermal fatigue, are the solder cracks between the copper base plate and the direct copper bonding (DCB) substrate and bond wire lift-off. Specific simulation tools are needed to carry out reliability researches and to develop device lifetime models. In other respects, accurate temperature and flux distributions are essential when computing thermo-mechanical stresses in order to assess the lifetime of high power modules in real operating conditions. This study presents an analysis method based on the boundary element method (BEM) to investigate thermal behavior of high power semiconductor packages subjected to power cycling loads. The paper describes the boundary integral equation which has been solved using the BEM and applied to the case of a high power IGBT module package (3.3 kV–1.2 kA). A validation of the numerical tool is presented by comparison with experimental measurements. Finally, the paper points out the effect on the thermal stress of the IGBT chips position on the DCB substrate. In particular, a light shifting of the silicon chips may be sufficient to delay significantly the initiation and the propagation of the cracks, allowing a higher device lifetime of the studied module.     

Thermal characterization of IGBT power modules

In this paper we report on experimental techniques for the thermal characterization of IGBT power modules. Three different systems have been used: the first one performs “in-time” characterization in order to control the most significant device parameters during normal operation or stress tests; the second one is for a complete and dynamic thermal characterization; finally, infrared optical analysis has been performed to validate the results.     

On the operation of a press pack IGBT module under short circuit conditions

Press pack insulated gate bipolar transistor (IGBT) modules connected in series for high-voltage direct current (HVDC) converter applications are designed such that when a failure occurs, it occurs in a safe manner by the formation of a stable short circuit, while redundant modules take up the voltage blocking function of the failed module. One such design using individual pressure contacts is described and the events occurring from the initiation of the short circuit to its final failure due to open circuit are reported from an electronics packaging materials design point of view. Experiments to hasten the failure under accelerated test conditions on modules were performed and interrupted at various stages of operation under a short circuit condition. The formation and subsequent aging of the metallurgical alloy under short circuit conditions was investigated by analyzing cross sections of the alloy forming the short circuit. Liquid metal corrosion along with the formation of intermetallics with poor conductivities lead to the final failure by open circuit.     

Reliability and lifetime evaluation of different wire bonding technologies for high power IGBT modules

IGBT modules for power transmission, industrial and traction applications are operated under severe working conditions and in harsh environments. Therefore, a consequent design, focused on quality, performance and reliability is essential in order to satisfy the high customer requirements. One of the main failure mechanisms encountered in high power IGBT modules subjected to thermal cycles is wire bond lift-off, which is due to the large thermal expansion coefficient mismatch between the aluminum wires and the silicon chips. The paper describes various bonding technologies using different wire materials directly bonded onto chip metallisation as well as the ABB solution where the wire is bonded on a thin molybdenum strain buffer soldered onto the chip. We assess in the present paper the potential of these technologies to enhance module reliability and lifetime through a power cycling test. Failure analysis results are presented and the failure mechanisms related to each technology are explained in detail.     

大功率IGBT功耗的研究

绝缘栅双极晶体管(IGBT)是复合全控型电压驱动式功率半导体器件.为了改善其功耗性能并进行进一步优化,论文在阐述IGBT特性基础上,通过从器件构成和实际应用角度对影响功率器件功耗的主要因素进行分析,并结合实践对IGBT功率器件的功耗进行深入研究,由此可以更深刻地理解IGBT功耗的产生,这对正确选择和使用IGBT器件及其系统有一定的实用价值.     

The high frequency behaviour of high voltage and current IGBT modules

Sharp voltage gradients act as a stimulus for high power IGBT modules, which can exhibit a potentially instable high frequency behaviour. In fact, they can act as a radio frequency amplifier and, in particular operating conditions, the interaction between the device and the control or the external circuit can cause self-sustaining oscillations or the enhancement of the unevenness in current distribution inside a power module thus having a significant impact on the reliability of the power converter. Moreover, this RF amplification worsen the generated EMI (Electro Magnetic Interference). This paper presents an extensive experimental investigation about the high frequency behaviour of IGBT high power modules. The measurements were performed by means of an original experimental set-up that was specifically conceived and constructed. The data are analysed with the help of a theoretical small signal model which is able to describe RF behaviour of high power IGBT modules.     

高压IGBT模块局部放电研究现状

IGBT(Insulated Gate Bipolar Transistor)模块一直朝着更高耐压和更大电流密度的方向发展,因模块内部电气绝缘和局部放电引起的问题也越来越明显。在高电压IGBT模块封装中,通常使用硅凝胶和环氧树脂来对模块进行灌注和密封,以满足其高电场承受能力,提升整个模块的绝缘性能和局部放电表现。目前很多国内外学者已经在这方面进行一系列研究,主要目的在于优化IGBT模块内部的电场分布。本文重点介绍目前研究的几种可以改善IGBT内部电场分布状态的方法,并对局部放电可靠性的提升方法进行总结。     

Effects of current density and chip temperature distribution on lifetime of high power IGBT modules in traction working conditions

This paper deals with ageing parameters of high power IGBT modules in traction applications. Using the results of a great number of power cycling tests on 400A modules, it shows that, in addition to the junction temperature excursion, other parameters like the maximal chip temperature and the current density are involved in thermal fatigue failures. Besides, contact temperature measurements achieved on IGBT chip surface in cycling conditions to localise the maximal thermomechanical stress are presented and correlated with modules failure analysis. The role of the current density in the ageing process is finally shown by determining its influence on the temperature gradient on chip surface.     

Temperature measurements and thermal modeling of high power IGBT multichip modules for reliability investigations in traction applications

To study the failure mechanisms induced on high power IGBT multichip modules by thermal cycling stress in traction environment, a good knowledge of the temperature distribution and variations on the chips and in the interfaces between the different layers of the packaging is necessary. This paper presents a methodology for contact temperature measurements on chips surface in power cycling conditions and a fast 3D thermal simulation tool for multilayered hybrid or monolithic circuits. The results of static and dynamic thermal simulation of a 1200A–3300V IGBT module are given and compared with the contact temperature measurements results. The investigation has been done within the RAPSDRA (Reliability of Advanced High Power Semiconductor Device for Traction Applications) European project.     

Active Voltage control of IGBTs for high power applications

The operation of an insulated gate bipolar transistor (IGBT) in its active region is a well established technique for withstanding short circuits and also for dv/dt control. In this paper, we exploit the active behavior of the IGBT, applying a voltage feedback loop to the IGBT to control its switching. It is shown that adding a bias to the demand reference waveform shifts the IGBT into the active region and permits wide bandwidth operation over most of the switching transient. The operation of the IGBT is reported in detail, making reference to a selection of experimental waveforms for 400-A, 1700-V capsule IGBTs. The implementation required for control of such large IGBT modules and capsule devices for high power applications is described and discussed. It is concluded that the active voltage control method allows the operation of high power IGBT circuits to be closely defined.     

对大功率IGBT模块损耗的研究

大功率IGBT模块损耗的确定是三相PWM变流器设计的一个关键参数。文中对两种类型转换器的功率半导体损耗进行了完整的分析计算。所提出的计算基础是将大功率IGBT的损耗简化为传导损耗和转换损耗。这种近似计算方法有助于预测和进一步研究两种变流器的功率IGBT的损耗性能。仿真计算结果指出了大功率IGBT损耗与变流器类型、工作点和脉冲宽度调制度的相关性,证明了所提出的计算方法的有效性。     

Reliability of AlN substrates and their solder joints in IGBT power modules

The reliability of IGBT modules was investigated with respect to the metallized ceramic (substrate) and the solder layer between the substrate and copper baseplate. Thermal cycles were performed between −55°C and +150°C on substrates based on different technologies and from various manufacturers. An incipient delamination of the metallization could be predicted from the mechanical resonance frequency. The warping of the substrates after cycling due to crack propagation and the adhesion of the metallization were determined. Thermal and active-power cycles were performed on 1200 A / 3.3 kV IGBT power modules to investigate the reliability of the solder joint between substrate and baseplate.