Stephane Azzopardi

Thermomechanical modelling and reliability study of an IGBT module for an aeronautical application

In this paper, an IGBT (Insulated Gate Bipolar Transistor) module for an aeronautic application was investigated using non-linear Finite Element Analysis (FEA). Two failure modes were considered: brittle materials fracture and solder joint thermomechanical fatigue. The module reliability regarding these failure modes was evaluated over the 40000 flight cycles required for the application, under the real loading profile. Acceleration Factors (AF) were also computed with respect to Accelerated Thermal Cycling (ATC) used during the module design qualification and validation.

Study of die attach technologies for high temperature power electronics: Silver sintering and gold-germanium alloy

Silver sintering joints and AuGe soldering are promising technologies for high temperature (>200 °C ambient) power electronics packaging. This paper presents the implementation of two silver-sintering processes with the one hand micrometer-scale silver particles, and on the other hand nano-meter-scale particles. Two substrates technologies have been investigated: A12O3 DBC and Si3N4 AMB. After the process optimization, tests vehicles have been assembled using both sintering processes, as well as a more classical high-temperature die attach technology: AuGe soldering. Multiple analyses have been performed, such as thermal resistance measurement, shear tests and micro-sections to follow the evolution of the joint during thermal cycling and high-temperature storage ageing.

Assessment of the Trench IGBT reliability: low temperature experimental characterization

The purpose of this study is an assessment of the Trench IGBT reliability at low temperature under static and dynamic operations by the aim of intensive measurements. The analysis of the Trench IGBT behaviour in these conditions is dedicated to the HEV applications. One question can be raised in case of the use of HEV in countries where during winter the temperature drops down -50°C or less: are Trench IGBT strongly affected by the low temperature environment? In this paper, we present experimental results under various test conditions (temperature, gate resistance, voltage and current) to give an understanding of the device behaviour by focusing on the device current and voltage waveforms and the power losses.

Assessment of uni-axial mechanical stress on Trench IGBT under severe operating conditions: a 2D physically-based simulation approach

Power devices in their environmental application are submitted in addition to the electrical, thermal, radiation and other kinds of stresses to mechanical stress. In our study, we focuse in combination of electrical, thermal and mechanical stresses of a Trench Gate Punch Through Insulated Gate Bipolar Transistor during switching transients. With a 2D physically-based finite elements simulator, we can achieve electro-thermo-mechanical simulations. Two operating configurations have been analyzed: inductive switching and short circuit. Simulation results show that whereas the mechanical stress has low effect on the current tail during clamped inductive switching turn off and on the sustain voltage during unclamped inductive switching, the saturation current during short-circuit operating is strongly affected by external mechanical stress depending on its level, direction and nature (compressive or tensile).

Processing and characterization of a 100% low-temperature Ag-sintered three-dimensional structure

Due to RoHS restrictions, researches on lead-free packaging have increased over the past decade. Low Temperature Joining Techniques (such as silver sintering or Ni-Au Transient Liquid Phase Bonding) are particularly studied because they are processed below 300°C and the attaches obtained are reliable at high temperature. Silver paste sintering technique for die backside attach is now a well-known technology used in both industry and academic research. The objective of this work is to adapt the procedure related to conventional silver paste sintered assembly to produce a three-dimensional module. For such purpose, this paper presents some theoretical considerations on silver sintering in order to briefly introduce the technique. Then, sintering procedures for realizing both die backside attach and three-dimensional structure will be described. Experimental results (electrical and mechanical tests, cross-sections, thermal measurements) will be given and performances of the double-side structure will be compared to die backside attaches and with lead-free solder. This work demonstrates for the first time the feasibility of a 100% silver sintered three-dimensional module using silicon diodes and Ni-Au substrate. Mechanical properties of the two attach-packages are better than classical solders and the performances of each attach are similar to silver sintered die backside attach. These promising results open the way to the use of silicon devices for high power density assemblies because the dice will be cooled down on both sides. This technology just requires silver metallization on the two sides of the device and can be extended to new materials such SiC or GaN for high temperature applications.

New Investigation Possibilities on Forward Biased Power Devices Using Cross Sections

For the first time, it is demonstrated in this letter that high-power silicon devices [diodes and insulated gate bipolar transistor (IGBTs)] can be forward biased and remains functional after cross sections. Sample preparation is presented, and electrical characterizations of a high-power diode and IGBT (600 V-200 A) have been performed in steady on-state. Infrared thermography on the cross-section surface using a macro-lens with high spatial resolution has allowed characterizing the vertical thermal distribution inside the power diode during forward bias. The impact of this work is that it opens a wide field of investigation in high-power semiconductor device characterization under forward bias.

A first approach on the failure mechanisms of IGBT inverters for aeronautical applications: Effect of humidity-pressure combination

Comparing to the most common reliability tests this work presents a new approach of accelerated testing, by combining temperature, humidity and pressure cycling with voltage stress. A design of experiments methodology has been proposed to test IGBT inverters and understand environmental factors effect. The humidity-pressure combination effect is studied for various IGBT inverter packaging materials.

Electromechanical characterization of “flying” Planar Gate Punch Through IGBT bare die

The effect of mechanical stress on the power dice is investigated by considering the application of mechanical stress on a Punch Through Planar Insulated Gate Bipolar Transistor under static electrical characterizations. Specific test vehicles and test bench for applying a tensile or compressive mechanical stress are described in detail. Furthermore, 2D finite element simulations are carried out to understand the device behavior by internal physics analysis. The impact of the mechanical stress cannot be neglected. Indeed, the results show a similar tendency between simulations and experiments for two static electrical characterizations: the output characteristics are sensitive to mechanical stress whereas the effect is negligible on the breakdown voltage. The case of the threshold voltage shows a disparity between experiments and simulations. Globally, such silicon property might be a key point to propose a real-time monitoring of the mechanical state evolution of the power assemblies.

Experimental electro-mechanical static characterization of IGBT bare die under controlled temperature

Silicon dice soldered in power assemblies have to withstand simultaneously electrical, thermal and mechanical stress. Mechanical stress is an important issue because it will directly impact on both the device behavior and power modules reliability. This paper focuses on the electro-mechanical static characterization of a planar gate IGBT by the help of experiments at controlled temperatures. A specific test bench is proposed to make the experiments on silicone bare dice. It can be highlighted that mechanical stresses have a strong influence on the electrical characteristics of IGBT and this effect is independent from the die temperature. These properties might be a key point to point out an early failure indicator to improve design of the power module.

Thermo-mechanical simulations in double-sided heat transfer power assemblies

In power assemblies, heat transfer due to the die self-heating is one of the most important point on time life assemblies. Heat has to be evacuated toward the base-plate not to weaken the solder joint under the die. Double-sided assemblies are attractive for heat transfer and many studies were initiated to have better heat transfer. So, we can observe less density energy deformation (DED) in solder joints and more stresses in the die. The purpose of this paper is to quantify the part of DED in the joint compared to the stresses in the die and finally to see the best configuration between single or double face assemblies.