• Sciences de l'ingénieur/Matériaux
  
• Sciences de l'ingénieur/Mécanique/Biomécanique
  
• Sciences de l'ingénieur/Mécanique/Mécanique des matériaux
  
• Sciences de l'ingénieur/Mécanique/Matériaux et structures en mécanique
  
• Sciences de l'ingénieur/Mécanique/Vibrations
  

The paper presents the impact of thermal cycling frequency on the lifetime of IGBT power module. The first part will be devoted to a quick presentation of the test bench. Then, the test protocols will be detailed as well as the measurement protocol to characterise the ageing of the power module wirebonds. An important part will be devoted to the results of ageing. Before concluding, mechanical tensile tests on bondwires will complete the ageing tests.

Ce poster présente un premier essai d'approche numérique du vieillissement des wirebonds dans les modules de puissance à semi-conducteurs qui tient compte des différents mécanismes de couplage multi-physiques qui se produisent au cours de la vie du composant. Les interactions électro thermiques et thermomécaniques sont décrites physiquement et modélisées numériquement. Les calculs montrent d'importantes déformations viscoplastiques dans les zones de connexion et au niveau des fils de bonding dès les premiers cycles de chargement. Une stabilisation quasi-cyclique des états mécaniques et thermiques est observée à plus de cent cycles (accomodation viscoplastique). Les mécanismes d'endommagement par fluage-fatigue deviennent alors prépondérants en induisant des couplages mécaniques-électriques.

Generally, studies on ageing of power module have very often sought to write phenomenological laws able to give an estimate of their lifespan. These laws, identified experimentally, are empirical functions of the characteristic electrical loads that the component must withstand. As an alternative, this paper presents a first attempt of numerical approach for wire bond ageing in semiconductor power modules that takes account for the different multi-physical coupling mechanisms that occur during the component life. The electro-thermal and thermo-mechanical interactions are physically described and numerically modeled. The calculations show important viscoplastic strains in the connection zones and bonding wires from the first loading cycles. A quasi-cyclic stabilization of the mechanical and thermal states is observed at more than a hundred cycles (viscoplastic shakedown). The creep-fatigue damage mechanisms become then preponderant inducing mechanical-electrical couplings.

Abstract Trees can generate large mechanical stresses at the stem periphery to control the orientation of their axes. This key factor in the biomechanical design of trees, named “maturation stress”, occurs in wood fibres during cellular maturation when their secondary cell wall thickens. In this study, the spatial and temporal stiffening kinetics of the different cell wall layers were recorded during fibre maturation on a sample of poplar tension wood using atomic force microscopy. The thickening of the different layers was also recorded. The stiffening of the CML, S 1 and S 2 -layers was initially synchronous with the thickening of the S 2 layer and continued a little after the S 2 -layer reached its final thickness as the G-layer begins to develop. In contrast, the global stiffness of the G-layer, which initially increased with its thickening, was almost stable long before it reached its final maximum thickness. A limited radial gradient of stiffness was observed in the G-layer, but it decreased sharply on the lumen side, where the new sub-layers are deposited during cell wall thickening. Although very similar at the ultrastructural and biochemical levels, the stiffening kinetics of the poplar G-layer appears to be very different from that described in maturing bast fibres. Highlight New insights into the changes in mechanical properties within the cell wall of poplar tension wood fibres during maturation have been obtained using atomic force microscopy.

A variety of techniques were used to investigate flax yarns sampled from four selected Italian paintings on canvas dated between the 17th and 18th centuries and compare them with a modern flax yarn. The goal was to establish their state of preservation and highlight the critical issues thanks to a combined approach that used SEM, atomic force microscopy, nanoindentation, nuclear magnetic resonance, second harmonic generation imaging microscopy and FTIR spectroscopy. The condition of the flax fibres was assessed, and the results showed that two of the four canvases were in a generally good state of preservation, while the other two exhibited signs of biological attack and brittleness due to a previous relining intervention.