Adhesion loss voids

Interfaces generally act as preferential condensation regions for diffusing species. Diffusion of species to the interface can weaken the interface. Precipitation of gas, incorporated into the film during deposition or in the substrate surface during cleaning, at the interface will reduce adhesion by forming voids at the interface. Diffusion and precipitation of lattice defects also forms voids at interfaces, which causes adhesion loss. 

LARC-TPI is a linear thermoplastic PI which can be processed ia the imide form to produce large-area, void-free adhesive bonds. Mitsui Toatsu Chemicals, Inc., has obtained Hcense to produce this product commercially for appHcations such as adhesives, films, mol ding compounds, etc. These are thermooxidatively stable and show essentially no loss ia weight at 300°C ia air. Weight loss does not exceed 2—3% after isothermal aging ia air at 300°C for 550 h. 

In practice, thermal cycling rather than isothermal conditions more frequently occurs, leading to a deviation from steady state thermodynamic conditions and introducing kinetic modifications. Lattice expansion and contraction, the development of stresses and the production of voids at the alloy-oxide interface, as well as temperature-induced compositional changes, can all give rise to further complications. The resulting loss of scale adhesion and spalling may lead to breakaway oxidation " in which linear oxidation replaces parabolic oxidation (see Section 1.10). 

Loss of theoretical adhesive strength can also arise from the action of internal stress concentrations caused by trapped gas and voids. Griffith11 showed that adhesive joints may fail at relatively low stress if cracks, air bubbles, voids, inclusions, or other surface defects occur as a result of the curing process. 

As a result of the loss of cell-matrix adhesion, epi-thehal cells, normally attached to the underlying matrix, become detached from the basement membrane [60]. In the urine of patients with "acute tubular necrosis" in native and transplanted kidneys, there is significant tubular cell shedding with up to 100% viability of voided tubular cells. 

Voids at the interface may be formed by the growth process during interface formation, by the accumulation of defects due to mass transport processes, or by the loss of material by diffusion. Voids at the interface are evidenced by low adhesion, high contact resistance, and possibly low thermal conductivity. Interfacial voids are studied by careful TEM sample preparation and by surface analysis of the failure surfaces after failure. 

Diffusion away from the interface can cause loss of adhesion. For example, in the chromium-gold metallization, heating in air above 200°C will cause the chromium to diffuse from the interface to the gold surface, where it will oxidize. The formation of this chromium oxide surface layer hinders TC bonding of wire leads to the surface and the loss of chromium from the interface leaves voids and decreases the adhesion.This outdiffusion of the interfacial material is dependent on the gaseous ambient and a non-oxidizing or vacuum ambient reduces the diffusion,... 

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