Georgia Techs Packaging Research Centers vision of System on Package (SOP) requires that the ball grid array (BGA) package be eliminated and the integrated circuit (IC) directly assembled on the printed wiring board (PWB). Flip-Chip on board (FCOB) emerges as a viable solution which meets the industry requirements of (i) increased I/O, (ii) increased functionality and (iii) improved performance at lower costs. Nevertheless flip-chip on board (FCOB) reliability continues to be an important concern in electronic packaging industry. Moreover transition to Pb-free solder for interconnects and continuously shrinking geometries result in new modeling challenges. In addition, the integrity of the intermetallics (IMCs) at the interfaces of the solder/PWB and solder/die is one of the determinant factors in the reliability and continuity of electrical signals in flip-chip interconnects. Pb-free solder studies for the flip-chip assembly studies are limited and simplified so far, not fully incorporating the effect of intermetallics in the reliability. New modeling challenges involve many details, from geometry to material properties. A brittle IMC will lead to a fracture at the interface. Also IMC thickness can cause the variation in stresses in the underlying layers, causing delamination. Moreover IMC morphology can also depend on the metal finishes on the PWB. In this work, a combined numerical and experimental program has been developed to address the challenges mentioned above. The flip-chip on board assembly is modeled in 3-D for reliability studies, taking into consideration material non linearities and a 104 order of geometric variation to capture the die size in mm to sub-micron intermetallic thickness. The study intends to determine the stresses induced at the critical interfaces under thermo-mechanical loading incorporating the intermetallic material properties. Various failure modes of these assemblies were studied. Experiments were carried out for comparative reliability studies of Pb-free solder with eutectic Pb-based solder. Intermetallic formation and growth are characterized during thermal aging and its effect on reliability is determined. Parameters affecting intermetallic like under-bump Metallurgy (UBM) thicknesses are varied and its effect evaluated. Moreover experiments with three new substrate pad finishes on PWB are carried out to evaluate them as an alternative to Electroless nickel immersion gold (ENIG) for new Pb-free solder. The final aim of this study is to reach a better understanding of the reliability issues in FCOB.

Les vibrations aléatoires sont les sollicitations les plus représentatives à celles qui se trouvent dans l'environnement réel. Elles sont souvent rencontrées dans l'environnement de fonctionnement automobile, aéronautique, spatial, ferroviaire et d'autres. La particularité des vibrations aléatoires est l'utilisation de son analyse par fatigue. Grâce aux propriétés statistiques des signaux aléatoires que les méthodes spectrales permettent d'estimer la moyenne du dommage d'une façon efficace et économique mais avec certaines limitations. Une étude numérique sur un système électronique embarqué accompagnée des essais expérimentaux ont été mis en place dans cette thèse pour présenter une démarche complète d'une analyse spectrale des vibrations aléatoires. Une modélisation par éléments finis a été réalisée ainsi qu'une technique du 'zoom structural' en analyse spectrale des vibrations aléatoires a été proposée dans ce travail avec la présentation des équations de cette technique. Une application numérique a montré la validité de la technique proposée en appliquant une étude de l'endommagement par fatigue par l'approche temporelle et par l'approche spectrale. Les résultats numériques de cette application mènent à une estimation d'un temps à l'initiation de la fissure très proche du temps de défaillance observé dans les résultats des essais expérimentaux. Cette technique montre la performance et l'efficacité des méthodes spectrales dans l'estimation du dommage par fatigue aléatoire. Une étude de l'optimisation d'un système soumis aux sollicitations aléatoires a été développée. L'objectif de cette étude est de rechercher la conception optimale du système soumis aux vibrations aléatoires en posant des limitations sur le dommage qui en résulte. Ce dommage estimé par les méthodes spectrales de Dirlik et de Single Moment ne doit pas dépasser un dommage cible pour assurer la fiabilité du système étudié. / Random vibrations are the most representative excitations that can be found in the real environment. They are often encountered in the automotive, aeronautical, space, railway and other operating environments. The special feature of the random vibrations is their important role in damage fatigue analysis. The spectral methods allow to estimate efficiently and economically the mean of the damage using the statistical properties of the random signals. A numerical study on an embedded electronic system with experimental tests was set up in this thesis to present a complete spectral analysis of random vibrations methodology. A finite element model was performed as well as submodelling technique in spectral analysis of random vibrations with associated equations was proposed in this work. A numerical application has shown the validity of the proposed technique by applying the fatigue damage study using the time-domain approach and the spectral approach. The numerical results of this application lead to the estimation of a crack initiation time very close to the failure time observed in the experimental tests. This technique shows the performance and efficiency of spectral methods in the estimation of random fatigue damage. The optimization of a system subjected to random excitations has been developed in this work. The objective of this study is to retrieve the optimal design of the system subjected to random vibrations with limitations on the resulting damage. This damage estimated by Dirlik and Single Moment spectral methods must not exceed defined target damage in order to insure the reliability of the studied system.