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The weak point for the standard power IGBT modules in terms of reliability is thermal fatigue in solder joints due to the thermal stress induced by constitutive materials with different coefficients of thermal expansion (CTE). So far, many researches are aimed at defining accurate finite element simulation with constitutive equations of materials behaviour and fatigue failure relation connecting the inelastic strain and the number of cycles before failure. Even if these relations can be clearly identified, we can see that the validation of the finite element model is difficult due to the scatter of input data. In fact, the fatigue life of solder joints strongly depends on geometric shape, solders behaviour (due to the process) and applied load. The aim of this paper is to estimate the probability of failure of power module with the structural reliability methods. Thus the geometric, materials and loading variables are considered as random variables and the failure mode is modelled with the called limit state function. The two methods, response surface method and neural network method, are used here to evaluate the reliability of the lead-free solder. The sensitivities of the mean and the standard deviation for each random variable have been evaluated.

In this paper, two IGBT modules assembling technologies with double side cooling capabilities and high level of integration were proposed for aeronautic applications after a state of the art and failures analysis. These technologies are compared using design of experiment based on non-linear finite element analysis with various materials, with respect to their potential failures under thermal and power loading profiles. The configurations optimizing the lifetime and reliability level were pointed out by loading profile and failure mode. Recommendations were then done in order to choose the optimal configuration of assembly for each application. Finally, these design rules were followed by the study of parts dimensions effects on the design outputs in order to help dimensioning the IGBT modules.

The aim of this work is to estimate errors related to Stereo-correlation technique applied to measurements of displacements. In a first step, the influence of the cameras positions is tested using a standard object. In a second step, an evaluation of the stereo-correlation system is achieved. It is based on the generation of synthetic images representative of real speckle patterns deformed. With these images, various parameters are treated in order to determine their impact on the measurement error.

Super-pressure balloons developed by the Centre National d'Etudes Spatiales (CNES) provide a cheap, non-polluting, and quickly operable platform for in-situ measurements at altitudes of up to about 25 km, and for remote sensing of the earth, the atmosphere, and space from regions inaccessible to other means of transport over similar observation periods. They are lifted by a helium-filled, approximately spherical, closed envelope made of a polyethylene terephthalate (PET)-based multi-layer polymeric film. It appears that during working flights, these balloons may sometimes explode prematurely. For this reason, the CNES is carrying out research programme and experiments in order to predict better stability of balloon flight and guarantee a life span corresponding to the technical requirements. In this article, experimental results are presented concerning the identification and characterization of film damage induced by simple and triple wrinkling. The present authors started from microscopic observations of damage involving wrinkles and from unidirectional tensile tests of the damaged film with simple and triple wrinkles. During tensile tests, in order to observe the zone affected by wrinkling, an optical technique was used (electronic speckle pattern interferometry) to measure local strain in the damaged area. The correlation between the strain level in the wrinkles observed during tensile testing and the evolution of damage parameters depending on strain gives the damage parameter in the case of simple and triple wrinkling. [ABSTRACT FROM AUTHOR]

Spherical plain bearings are used on many components of Airbus aircrafts like engine-to-pylon and pylon-to-wing links. Their design is based on contact pressure distribution on spherical surfaces. Until now, empirical laws are applied to determine it, and the aim of this work is to develop a numerical modelling in order to validate the use of these laws. The assumptions on the numerical model and their significance have been verified and meas-ured by scattered light photoelasticity (SLP) on an epoxy replica of the bearing. The analysis of this type of bearing is difficult due to its three-dimensional behaviour and its spherical confined contact. The SLP technique allows getting information on stress fields inside an epoxy specimen with the analysis of photoelastic fringes. Comparing photoelastic fringes with simulated results, the finite-element model has been improved to be more representative of the real life bearing. In addition, some critical factors on the bearing behaviour have been obtained. [ABSTRACT FROM AUTHOR]

Long duration super-pressure balloon design and manufacturing is a great challenge for the Centre National d'Etudes Spatiales in scientific ballooning. They are made of 50 μm thick polymer films polyethylene terephtalate/polyamide/polyethylene terephtalate which are joined with 23 μm thick adhesive polyethylene terephtalate bands. To date, during flight missions, a major problem appears: some balloons explode prematurely. This is why this study deals with the mechanical behaviour of the stratospheric pressurized balloons and particularly with the assembly of the constitutive films material. First, a nanoindentation test campaign makes it possible to identify the elastic properties of the materials at two different temperatures. Then, a nominal assembly is simulated in two stages: two-dimensional and three-dimensional. Finally, the factorial design method combined with finite-element simulations was used to study the influence of the various conception parameters on the mechanical behaviour of the assembly. [ABSTRACT FROM AUTHOR]

A new ohmic contact technology on diamond with low resistivity is presented. A Si/Al metallisation is used and leads to a 7×10-6 Ω.cm2 specific contact resistivity, measured by the transfer length method. Comparison is made between this technology and the Ti/Pt/Au traditional technology. Both technologies show very good electrical, thermal and mechanical characteristics. [ABSTRACT FROM AUTHOR]

An experimental investigation of two potential candidate materials for the diamond die attachment is presented in this framework. These efforts are motivated by the need of developing a power electronic packaging for the diamond chip. The performance of the designed packaging relies particularly on the specific choice of the solder alloys for the die/substrate junction. To implement a high temperature junction, AuGe and AlSi eutectic alloys were chosen as die attachment and characterized experimentally. The choice of the AlSi alloy is motivated by its high melting temperature T m (577 °C), its practical elaboration process and the restrictions of hazardous substances (RoHS) inter alia. The AuGe eutectic solder alloy has a melting temperature (356 °C) and it is investigated here for comparison purposes with AlSi. The paper presents experimental results such as SEM observations of failure facies which are obtained from mechanical shear as well as cyclic nano-indentation results for the mechanical hardening/softening evaluation under cyclic loading paths. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, errors related to digital image correlation (DIC) technique applied to measurements of displacements are estimated. This work is based on the generation of synthetic images representative of real speckle patterns. With these images, various parameters are treated in order to determine their impact on the measurement error. These parameters are related to the type of deformation imposed on the speckle, the speckle itself (encoding of the image and image saturation) or the software (subset size). [Copyright &y& Elsevier]

Abstract: The weak point for Insulated-Gate Bipolar Transistor (IGBT) modules in terms of reliability is thermal fatigue in solder joints due to the thermal stress induced by constitutive materials with different coefficients of thermal expansion (CTE). Now, many researches aimed to define accurate finite element simulation with constitutive equations of material behavior and fatigue failure relation connecting the inelastic strain and the number of cycles before failure. Even when these relations are clearly identified, the validation of the finite element model is difficult due to the scatter of input data. In fact, fatigue life of solder joints strongly depends on geometric shape, solders behavior (due to the process) and applied load. The aim of this article is to estimate the probability of failure of power module with structural reliability methods by considering geometric, material and loading variables as random variables. Since in a non-linear context, the finite element calls are expensive in terms of computer run time, an FE strategy is proposed here to replace conventional 3D mesh of layer by 3D-shell. To reduce computation time, response surface method, which approximates the output strain with respect to input random variable (RV) with the design of experiment (DOE) procedure, is used to perform reliability analysis. This reponse surface allows at the end to perform Monte Carlo random simulation process for fitting Weibull and fatigue life distribution on the output inelastic strain. [Copyright &y& Elsevier]

In this paper a two-dimensional and a three-dimensional finite element models of unsteady-state metal cutting are presented. These models take into account dynamic effects, thermo-mechanical coupling, constitutive damage law and contact with friction. The simulations concern the study of the unsteady-state process of chip formation. The yield stress is taken as a function of the strain, the strain rate and the temperature in order to reflect realistic behavior in metal cutting.Unsteady-state process simulation needs a material separation criterion (chip criterion) and thus, many models in the literature use an arbitrary criterion based on the effective plastic strain, the strain energy density or the distance between nodes of parts and tool edge. The damage constitutive law adopted in models presented here allows defining advanced simulations of tool''s penetration in workpiece and chip formation. The originality introduced here is that this damage law has been defined from tensile and torsion tests, and we applied it for machining process. Stresses and temperature distributions, chip formation and tool forces are shown at different stages of the cutting process.Finally, we present a three-dimensional oblique model to simulate the unsteady-state process of chip formation. This model, using the damage law defined before, allows an advanced simulation close to the real cutting process. The final part shows a milling application.An arbitrary Lagrangian Eulerian formulation (ALE) is used for these simulations; this formalism combines both the advantages of Eulerian and Lagrangian representations in a single description, it is exploited to reduce finite element mesh distortions. [Copyright &y& Elsevier]

Mechanical suitability of ohmic contacts among the select metallization systems, deposited on a p-type heavily boron-doped homoepitaxial diamond layer, was evaluated via mechanical tests on the nanoscale. Two candidate metallization systems were considered: Si/Al and Ti/Pt/Au. Metallizations were performed using two different techniques: plasma-enhanced chemical vapour deposition and “lift-off”. Effectiveness of the techniques was assessed via mechanical tests on the microscale and the nanoscale. Nano-indentation experiments were performed to determine the mechanical properties of the layers. Nano-scratching experiments were used to evaluate the mechanical adhesion on the diamond substrate. Scanning electron microscopy was applied for observation of the morphology of the surface and the indent and for detecting defects. [ABSTRACT FROM AUTHOR]

Abstract: This work examines the thermal fatigue effects on different configurations of power modules used in harsh aeronautical environment. They are used in various applications where the temperature cycling due to the working environment is the most limiting fact. In this case, it is highlighted that the topology assembly choice is a critical point to reach the lifetime required for the final application. In addition, it is proposed to correlate the probabilistic finite elements calculus to the experimental accelerated ageing tests on test vehicle, in order to determine the best configuration of assembling stack consisting of baseplate/RoHS solder/metallized substrate. [Copyright &y& Elsevier]

Abstract: In order to predict the flight parameters and to improve the life time of long duration super-pressure balloon, a research program on modelization and experimental simulation of the balloon envelope mechanical behavior is carried out. The balloon is a 10m diameter type made with multilayer polymeric films and tapes. A facility was developed to measure, via a stereo-correlation system, the 3D displacement and the in plane solicitation strain of a 1.5m diameter balloon envelope part in realistic flight conditions, i.e. pressure, temperature and loading at the sample boundaries. A time dependant non-linear Maxwell model of the polymeric material behavior was identified from uniaxial creep and relaxation tests and implemented in a Finite Elements code, simulating the sample tested in the facility. The Poisson ratio of the transparent and supple balloon film has been measured with an image correlation system. Experimental results are obtained both at room and in cold conditions (−60°C) for various values of differential pressure. Vertical displacement and in plane 2D strain of apex part deduced from the numerical modeling are compared to experimental results. [Copyright &y& Elsevier]