Failure modes residual stress

Film Adhesion. The adhesion of an inorganic thin film to a surface depends on the deformation and fracture modes associated with the failure (4). The strength of the adhesion depends on the mechanical properties of the substrate surface, fracture toughness of the interfacial material, and the appHed stress. Adhesion failure can occur owiag to mechanical stressing, corrosion, or diffusion of interfacial species away from the interface. The failure can be exacerbated by residual stresses in the film, a low fracture toughness of the interfacial material, or the chemical and thermal environment or species in the substrate, such as gases, that can diffuse to the interface. 

The standard mechanical tests, as described in Section 7. can normally be undertaken with care for composites as a function of temperature. The difference between fiber- and matrix-dominated properties can result in different temperature dependencies. Changes in the residual thermal stresses present can occur both between the fibers and the resin, and between layers, in particular between 0" and 90 orientated unidirectional layers. Care needs to be taken in assessing the failure mode, particularly in flexural and compressive tests where there can be changes, particularly at elevated temperatures, due to the matrix providing a lower degree of support to the fibers, thus encouraging compression failure. 

Besides delamination, actual cracking of silicon die is a failure mode that can occur from excessive adhesive stresses, voids, and moisture absorption. Residual stresses in adhesive-attached single-chip devices become critical as the size of the device increases and dissimilar die and leadframe materials are used. Several types of fractures that can occur within the die or within the adhesive are shown in Fig. 6.8. A hairline crack in an 1C chip that resulted from adhesive stress is shown in Fig. 6.9. 

As seen, even relatively crude analytical energy balance models are helpful in understanding crack stability in adhesive layers. This energy-based perspective provides an intuitive understanding of the effect of in-plane stresses within an adhesive layer on crack path selection. Indeed, residual stresses have been shown to induce some very interesting failure modes in bonded joints, including spiral crack patterns [63]. 

Hydrogen-Induced Cracking (Hic) This mode of failure of a component involves hydrogen atoms absorbed in the steel leading to the development of internal cracks in low-strength steels. The cracks generally lie parallel to the rolling plane and the surfaces of the steel component. Residual or applied stress is not needed for HIC development. 

Film adhesion may fail spontaneously without the application of any stress. This may be due to very poor adhesion or high residual film stress.Figure 11.1 depicts several modes of failure. 

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