Denis Bertheau

Mechanical Properties of Sintered Ag as a New Material for Die Bonding: Influence of the Density

Sintered silver powder is a good candidate for die bonding as an alternative to lead alloys. However, little is known about its intrinsic mechanical properties, which are required to model the entire electronic system reliability, especially during ageing. Since interconnections are obtained by sintering, the mechanical properties are expected to be impacted by the residual porosity. However, evaluating the density of the material in its 30-μm-thick form is challenging. This issue was addressed by developing a robust procedure to obtain bulk specimens with the same microstructure as the interconnection. Tensile tests were performed at room temperature to determine the macroscopic characteristics (Young’s modulus, ductility, yield stress, ultimate tensile stress) as a function of the porosity.

Influence of the Porous Microstructure on the Elastic Properties of Sintered Ag Paste as Replacement Material for Die Attachment

Silver pastes are good candidates as alternative materials to lead solder alloys. However, little is known about the relationship between their microstructure and their mechanical properties. This issue is addressed by developing a specific route to obtain standalone sintered bulk specimens representative of the real sintered joints. The relationship between the density and the pore surface fraction is established, allowing the density of the material to be obtained independently from its size and geometry. The elastic constants of both sintered joints and sintered bulk specimens are investigated using dynamic resonant testing. A strong correlation between the elastic constants and the density is established. In contrast to the sintered bulk specimens, for which the Young’s modulus remains constant after annealing, Young’s modulus of the sintered joints evolves significantly towards a stabilized value. This is derived from thermal stresses relaxation within the sintered joint.

Anisotropy in creep properties of DS200 + Hf alloy

The directionally solidified Ni-based superalloy DS200 + Hf is used in aero-engines and industrial gas turbines for the manufacturing of intermediate and low-pressure turbine blades. This material consists of ~<0 0 1> columnar grains oriented along the solidification direction and with random secondary orientations. In this study, the creep behaviour of the DS200 + Hf alloy has been investigated along longitudinal and transverse directions. The main objective was to investigate the durability of this alloy and the damage mechanisms for four different crystalline microstructures: DS200 + Hf composed of fine grains loaded longitudinally, the same alloy composed of coarse grains or fine grains, loaded transversally to the solidification direction and finally, the single crystal version, Mar-M200 + Hf, loaded along a <0 0 1> crystallographic orientation. Creep tests were performed from 750°C up to 1100°C. Under all conditions, a pronounced decrease in the strain to failure is observed for specimens loaded along the transverse direction compared to specimens tested along longitudinal direction or single crystals. Fracture surface observations revealed that the lower creep ductility along such a loading direction results from an intergranular failure mode. Furthermore, the creep life anisotropy significantly decreases when temperature increases. Additional creep experiments under vacuum at 900°C and experiments using single crystalline specimens far away from the <0 0 1> crystallographic orientation show a prominent damaging role of oxidation at very high temperature and a major contribution of the local crystallography at low temperatures. In addition, the grain size effect observed when testing in the transverse direction only results from the grain boundary oxidation.

Determination of the tensile residual properties of a wound carbon/epoxy composite first exposed to fire

The use of carbon fiber/epoxy matrix composite is widely developed to store hydrogen at high pressure because of its low weight and its good specific mechanical properties. In order to secure this type of storage, it is necessary to tackle the thermal degradation and the influence of a fire or a heating source on the residual mechanical behavior of such materials. In the present study, carbon/epoxy composite samples with different fiber orientations are considered. The thermal aggression (representative of a fire) is performed by using a cone calorimeter apparatus (ISO 5660). The fire exposure is stopped at different time in order to study the influence of the thermal energy (different heat fluxes and exposure durations) on the residual mechanical tensile properties. The results obtained show that the residue thickness (char) of the samples is proportional to the incident energy. Strength and stiffness reduction can be observed even without ignition (i.e., without combustion flame) when the mechanical properties are controlled only by the resin (fiber perpendicular to the loading axis). When the fibers are mechanically loaded (quasi-isotropic samples or ± 45° samples), a very little strength decrease is observed before ignition and accelerated after ignition. A proportional relationship between the ultimate stress of the exposed sample and the non-charred thickness is also observed.

Effect of a coupled thermomechanical loading on the residual mechanical strength and on the surface temperature of wound carbon/epoxy composite

Wound composite structures such as hyperbaric hydrogen tanks may experience accidental situations, for example in case of a fire. The FCH-JU project FireComp aims at better characterizing the conditions that need to be achieved in order to avoid a failure of a composite pressure vessel. This research program involves specific experiments to improve the understanding of loss of strength of composite high-pressure vessels in fire conditions. The present study investigates the effect of a coupled thermomechanical loading (cone calorimeter exposure and, simultaneously, mechanical stress) on the residual strength of a composite material. A specific device combining a cone and a four-point bending bench has been designed. The influence of the coupled aggression is addressed by comparing the temperature on the front and the rear sides, the mass loss, and the residual tensile strength of a set of samples subjected to a heat flux only and a set subjected to a heat flux and a four-point bending. The results do not exhibit a clear effect of the mechanical load: the thermomechanical properties of both sets of samples are similar.

Crack Growth in a Parallelepipedic Specimen Subjected to a Cyclic Temperature Gradient

Abstract A parallelepipedic coupon containing an edge notch has been submitted to cyclic edge temperature evolution, implying a cyclic temperature gradient in the specimen thickness. Associated analytical calculation of the stress intensity factor is also carried out using Duhamels theory and weight functions: this allows the study of the influence of geometric and thermal parameters on crack development. Moreover, an automatic procedure using scripting language introduced in a tridimensional numerical ABAQUS simulation leads to a prediction of the crack shape evolution in good agreement with experimental observations.