Materials Science Forum Vols. 524-525

Abstract: In this paper we will discuss the impact of residual stresses on the reliability of microelectronic components and the materials used therein. The following issues will be particularly emphasized: First, the tendency toward delamination and subsequent cracking along interfaces, such as between silicon dies, organic substrates, glues, and underfill material; second, the fatigue of electrolytically deposited copper vias within the substrate and FR4 board material; third, the accumulation of irreversibly accumulated plastic (creep) strain in lead containing as well as leadfree solders; the microstructural change observed during thermo-mechanical use within the bulk as well as at the interface of solder interconnects. We will present state-of-the-art numerical techniques that allow to quantify the development of stresses and strains within the aforementioned materials, mostly by finite element analysis, as well as the coupling between local stresses and diffusion processes, which is theoretically based on phase field models. Further emphasis is put on proper knowledge and determination of the inherent material parameters and how theoretical predictions can be linked to and validated by experimental observations and facts.

Abstract: A rigorous strategy for (X-ray) diffraction stress measurements at fixed penetration/information depths is described. Thereby errors caused by lack of penetration-depth control in traditional (X-ray) diffraction (sin2ψ) measurements are annulled. The ranges of accessible penetration/information depths and experimental aspects are briefly discussed. The power of the method is illustrated by the analysis of an only small stress gradient in a sputter-deposited nickel layer.

Abstract: Stress gradients have been investigated employing a measurement strategy for diffraction measurements at constant penetration/information depths. Two examples have been considered: (i) sputter-deposited copper thin films on silicon wafers and (ii) γ’-Fe4N1-x layers on α-Fe substrates obtained by gaseous nitriding. In the Cu thin films rather low tensile stresses, increasing in magnitude with increasing penetration/information depth have been found. An evaluation of the measured lattice strains has been performed on the basis of the f(ψ) method, where the X-ray elastic constants (XEC’s) have been calculated as weighed averages of the corresponding Voigt and Reuss XEC’s and the weighing parameter has been taken as a fitting parameter. This evaluation reveals that the grain interaction changes with increasing penetration/information depth from near-Reuss type towards Neerfeld-Hill type. In the γ’-Fe4N1-x layers stress gradients occur due to surface relaxation near the surface and deeper in the layer due to a nitrogen concentration gradient which is built up during nitriding. First measurements in a laboratory diffractometer show the effect of surface relaxation on the stress-depth profile near the surface. As no single-crystal elastic constants are available for γ’-Fe4N1-x, the mechanical elastic constants have been employed in diffraction stress analysis. The results indicated that single-crystal elastic anisotropy occurs. From the measured data also a concentration – depth profile has been deduced.

Abstract: A new algorithm is proposed to determine the through-thickness residual stress gradient by X-ray Diffraction measurements on progressively thinned components. The procedure is based on a chemical or electrochemical attack of the component surface, which is then measured at each thinning stage. The simple algorithm provided for by a specific norm has been revised to take into account the X-ray absorption effects and the conditions of mechanical equilibrium of the component. The new procedure is illustrated for a typical case of study concerning a shot-peened metal component.

Abstract: The detection of near surface residual stress gradients in real space requires high depth resolution for any orientation of the diffraction vector with respect to the sample co-ordinate system. In order to meet this demand, the slits are no longer being fixed in the laboratory co-ordinate system as in strain scanning experiments but directly coupled with the sample. Hence, the gauge volume orientation within the sample remains constant and allows performing depth-resolved sin2ψ measurements in real space. The method’s accuracy is determined by the gauge volume definition, which is investigated in detail. Apart from the evaluation of the σ(τ) versus σ(z) relation, which is of fundamental interest in X-ray residual stress gradient analysis, the method will be shown to have a unique applicability in rather delicate sample geometries such as multilayer systems.

Abstract: In this work, the effect of steel grade on the fatigue resistance of deep-rolled crankshafts is analysed. In the first part of this paper, the mechanisms leading to the increase of the fatigue resistance brought by the deep rolling treatment, is presented. This reinforcement is mainly linked to crack arrest due both to a decrease of the in-depth stress concentration factor and to remaining compressive residual stresses induced by the deep rolling. In a second part, an analytical model of residual stresses generation by deep-rolling and fatigue is presented. In this model the low cyclic fatigue behaviour of the steel is taken into account, and the residual stress stability with bending fatigue cycling can be predicted. After a presentation of the experimental validation on two different microstructures (baintic and ferrito- perlitic), this model is used for analysing the main parameters of the deep-rolling process and fatigue resistance.

Abstract: In the work presented here the residual stress states of sintered iron (ASC 100.29) were studied after mechanical surface treatments. This included the investigation of the stability of compressive residual stresses of deep rolled sintered iron at different angles relative to the rolling direction at quasistatic and cyclic bending loading. An increase of the compressive residual stress in the transversal deep rolling direction at fatigue loading was found and will be discussed in this presentation.

Abstract: Different classes of metallic materials (aluminum alloys, steels, titanium alloys) were mechanically surface treated by deep rolling and laser shock peening and isothermally fatigued at elevated temperature under stress control. The fatigue tests were interrupted after different numbers of cycles for several stress amplitudes and residual stresses and FWHM-values were measured by X-ray diffraction methods at the surface and as a function of depth. The results summarize the response of the surface treatment induced residual stress profiles to thermomechanical loading conditions in the High Cycle Fatigue (HCF)- as well as in the Low Cycle Fatigue (LCF) regime. The effects of stress amplitude, plastic strain amplitude, temperature and frequency are addressed in detail and discussed. The results indicate that residual stress relaxation during high temperature fatigue can be predicted for sufficiently simplified loading conditions and that thermal and mechanical effects can be separated from each other. A plastic strain based approach appears to be most suitable to describe residual stress relaxation. Frequency effects were found to be not very pronounced in the frequency range investigated.

Abstract: Deformation by rolling induces in general a strong crystallographic texture, hence an important material anisotropy. This is a reason why the cross-rolling is sometimes applied in order to symmetrize the crystallographic texture. Such an operation modifies also residual stresses. The goal of this study was to characterize residual stress and texture changes during simple and crossrolling in polycrystalline copper and ferritic steel. The obtained results show that important modification of the first order residual stresses occurs during cross-rolling, while the level of the second order ones is approximately constant. Experimental results were analysed using an elastoplastic deformation model.