Philippe Pougnet

Probabilistic fatigue damage estimation of embedded electronic solder joints under random vibration

Abstract The stress response to random vibrations is an important factor to be taken into account in designing embedded electronic devices. Several test specifications and qualifications use the random vibrations to increase the reliability of electronic products. Fatigue damage estimation of embedded electronic solder joints has not been addressed, especially under random vibration loading. In this paper numerical random vibration analysis with finite element method of an embedded electronic device is used to estimate the stresses Power Spectral Density (PSD). These PSD of stresses are then used in different probabilistic fatigue damage approaches to estimate the reliability of the solder joints regarding the vibration fatigue damage. Two different probabilistic approaches of random fatigue are employed; the time-domain approach based on Rainflow and Monte Carlo simulations and the frequency domain approach based on spectral techniques and statistical data. However, the fatigue damage estimation depends mainly on the accurate results of the stresses PSD. Thus, this work proposes to develop the sub-modelling technique to the random vibration analysis in order to provide accurate results of the stresses PSD. Moreover, this paper discusses the advantages of the approaches listed above to estimate the fatigue damage in solder joints of an electronic component and shows the effectiveness of sub-modelling techniques in the random vibration analysis.

Reliability-Based Design Optimization

In order to increase and maintain their businesses, mechatronics manufacturers develop innovative products and reduce product development costs. Economic constraints motivate them to reduce the duration of the testing phase and the number of prototypes and to develop simulations. Introducing innovations enables them to meet customers’ expectations and stand out from their competitors. However the risk that their products might not function correctly in operational conditions must be evaluated with precision as defects during the warranty period have a negative impact on the bottom line. To reduce these industrial and financial risks and respect the levels of performance and reliability required, reliability actions need to be part of design. This chapter describes a reliability-based design optimization approach which is adapted to mechatronics systems. The first step involves establishing predictive reliability of the electronics system in operational conditions based on the FIDES reliability manual. This recent manual based on probability laws is regularly updated according to field returns. The objectives of the following steps are to identify the potentially faulty elements in the life profile conditions and to establish stress distributions leading to failures. In order to understand failure mechanisms, the effects of mechanical, thermal and electromagnetic loads on several prototypes are characterized experimentally. After this, tests are used to provoke failures. Consecutive failure analysis helps to develop multiphysics failure models. These models are optimized and validated by comparing model responses to thermal or vibratory solicitations and those obtained experimentally. Developing meta models capable of treating the variability of life profile loads and of fabrication enables reliability predictions. Design is then optimized by adjusting the architecture elements which are critical for reliability.

10 – Study on the Thermomechanical Fatigue of Electronic Power Modules for Traction Applications in Electric and Hybrid Vehicles (IGBT)

Abstract: The reliability of the power module is mainly related to that of its electronic components (IGBT power chips and diodes). These components undergo severe and varied stresses. Indeed, the electrical pulses due to the internal operations of these components cause thermal loadings inducing mechanical deformations which can go beyond the thresholds desired. Furthermore, these electronic components are subjected to vibrations which can affect the state the solder joints and the electrical wire connections, causing damage by thermomechanical fatigue. To this end, knowledge of the mechanical response of the power module is vital for the electronics industry as these modules fulfill strategic functions, as is the case for electric and hybrid vehicles. Knowledge of the mechanical behavior of this power module requires the modeling of several physical phenomena. Multi-physical modeling aims at considering the interdependencies and interactions between different physical phenomena such as electrical, thermal and mechanical. In this study, multi-physical coupling is performed using ANSYS software.

Estimation of Fatigue Damage of a Control Board Subjected to Random Vibration

Abstract: On-board electronic systems are often exposed to different types of loading due to their operating environment. These loadings are represented mainly by thermal and vibration effects. In this chapter, the study is limited to vibration loads. In the framework of the FIRST-MFP project, an industrial application, the Valeo S97 demonstrator, of the effect under the influence of random vibrations is measured. The behavior of the control board for the DC–DC converter inverter of a hybrid vehicle is the subject of this study. The objective of this chapter is to estimate the fatigue damage at the soldered joints. The random stresses and the properties of the materials present a wide range of uncertainties. Experimental tests as well as numerical simulations will make it possible to study the uncertainties that influence the behavior of the structure.

Monte Carlo Method Applied to the ABV Model of an Interconnect Alloy

A Monte Carlo (MC) simulation of a 2D microscopic ABV (metal A, metal B and void V) Ising model of an interconnect alloy is performed by taking into account results of Finite Element methods (FEM) calculations on correlated void-thermal effects. The evolution of a homogeneous structure of a binary alloy containing a small percentage of voids is studied with temperature cycling. The diffusion of voids and segregation of A type or B type metals is a function of the relative interaction energy of the different pairs AA, BB, AB, AV and BV, the initial concentrations of A, B and V and local heating effect due to the presence of clusters of voids. Voids segregates in a matrix of A type, of B type or AB type and form large localized clusters or smaller delocalized ones of different shapes.

Highly Accelerated Testing

“Highly accelerated life testing” (HALT) was invented in the USA in the 1980s. In Europe, this method is called both HALT and “highly accelerated testing” (HAT). HALT and HAT are experimental tests which reveal design weaknesses of electronic devices by subjecting them to vibration, temperature and ramp temperature stresses. These tests are best used at technology readiness level 4 or 5 in product development as specified in the international norm ISO 16290. HAT tests take place in a dry environment. Humidity is an important factor of stress which may lead to failures in embedded mechatronic systems. This chapter will present an HAT method in a humid environment. The principle is to apply humid air to the device under test (DUT). The HAT chamber makes it possible to vary the temperature rapidly at the same time as vibrations resulting in the humidity penetrating the DUT, especially when the sealing is defective. Depending on the temperature, this humidity takes the form of vapor or ice on the electronic boards and exposes the weaknesses of assemblies, interconnects or tightness defects. The design defects of the electromagnetic compatibility (EMC) circuits are revealed by performing conducted and emitted radiation tests before and after HAT.

Impact of voids in interconnection materials

This chapter deals with the study of heat transfer phenomena related to the dissipation of thermal energy in a power module and its subsequent effects. In the packaging process of a mechatronic module, defects may appear in the form of voids inside the interconnection material (ICM). By trapping thermal energy, these voids can act as a potential source of failure in the module. Indeed, they introduce an inhomogeneous character into the distribution of temperature at the interface of the different assembled components and within the ICM, and the resulting thermomechanical stresses weaken the packaging. Because a theoretical study of these effects is difficult, they are simulated using finite element methods (FEMs). The temperature distribution in the power module was determined by considering different types of ICMs, Sn60Pb40, Sn95,5Ag3Cu0,5, Ag and sintered Ag. Then, the thermomechanical stresses were simulated to determine the temperature effects on the packaging when voids are present in the ICM. This study aims to determine the influence of voids on the reliability of a module by using the parameters which are the maximum temperature of the chip and the thermomechanical stresses at the interfaces as indicators. This chapter gives an example of the importance of simulation in the design of a reliable mechatronic device. We not only obtain results which allow quick anticipation of the root cause of failures, but also significant data to optimize the bench tests to be implemented in order to improve the fabrication process and increase efficiency.

Reliability Prediction of embedded Electronic Systems: The FIDES Guide

Abstract Reliability prediction calculations usually take place in the first phases of the design process. These calculations are performed to verify that the chosen architecture will achieve reliability goals to identify the critical components for reliability and to evaluate the effect of the operating conditions on the failure rate of the components and, if necessary, to establish the renewal rate necessary for operational condition maintenance. Two reliability prediction handbooks are widely used as standards in the electronics industry: the MIL HDBK 217F and the UTE C 80-810. These manuals which are based on statistical assessments of field returns are no longer updated. The French Ministry of Defense (Direction Generale pour l’Armement, DGA) encouraged a consortium of French companies in January 2004 to develop a more precise reliability handbook: the FIDES guide. This manual is updated periodically in order to cover technological evolutions to extend its scope and to include any improvements. The most recent version is the FIDES 2009, which is recognized as the UTE C 80-811 standard. A precise and inclusive definition of the life profile, such as the operating conditions and environmental stresses of systems, has made it unique. Finally, the predictive models developed for the various families of components follow the state of the art as technologies evolve. In this chapter, FIDES is applied to predict the reliability of an embedded automotive mechatronic system.

Non-Destructive Characterization by Spectroscopic Ellipsometry of Interfaces in Mechatronic Devices

Abstract Failures often originate at the interfaces in the packaging of mechatronic devices because of the differences in the thermo-mechanical properties of the materials in contact. To enhance the reliability of these devices, it is necessary to have nondestructive analysis techniques such as spectroscopic ellipsometry (SE) to probe the quality of the surfaces and interfaces when the environmental constraints vary. In the field of characterization spectroscopy, SE has become indispensable in microelectronics, as well as in the study of semiconductors, protective coatings based on polymers, metals or other types of metamaterials. It is used to characterize thin films, mono- or multi-layers and bulk materials from a structural and optical point of view by probing transitions whether electronic, vibrational or rotational. In the ultraviolet (UV)- visible and near- to mid-infrared (IR) ranges, which correspond, respectively, to the electronic and vibrational absorption, SE reveals the composition, structure (amorphous or crystalline), porosity and morphology (density, roughness, etc.) of materials as a function of the light wavelength. In order to be used, SE parameters generally require an inverse method based on the simplex, the Levenberg–Marquardt or the Broyden– Fletcher–Goldfarb–Shanno algorithm to determine the dielectric function or complex optical constants. This chapter describes the SE technique and illustrates its application with two examples of characterization, that of sintered silver and polymers present in a mechatronic device. A study of the effects of temperature in dry and wet conditions is also presented and discussed in terms of optical properties.

Reliability in Mechatronics Systems from TEM, SEM and SE Material Analysis

Mechatronic systems designed to comply to new EU directives are studied through interconnections by electronic or photonic probes, SEM, TEM, SE or 3D Tomography. Leaded and lead free modules assembled by standard interconnection technologies are studied for robustness relative to thermal accelerated life tests. Results obtained from JEOL 6060LV SEM and Optical Microscopy show that although slow growth rate of inter-metallics (IMC) is consistent with expected reliability, they are responsible for propagation of cracks especially in the presence of gold on PCB side. Innovative Low Temperature Joining (LTJ) technology applied to nano or micro silver pastes which should reduce IMC effects are tested on mechatronic systems. Results obtained from SEM, TEM and 3D Tomography will be shown as well as non destructive Spectroscopic Ellipsometry studies of samples. Pressureless LTJ technology is unsuitable for robust interconnection.