Variations in the electrical and mechanical properties of silsesquioxane spin-on-glass thin films are examined as a function of curing time and temperature. Particular attention is paid to the tradeoff between producing low-dielectric-constant films, suitable for advanced microelectronic interconnection structures, and mechanically stable films, able to withstand semiconductor wafer fabrication processes. Two critical aspects of the mechanical stability of spin-on glasses are shown to be the positive thermal expansion mismatch with silicon, leading to tensile film stresses, and reactivity with water, leading to susceptibility to stress-corrosion cracking. An absolute reaction-rate model is used to predict crack velocity using a deleted-bond model and fused silica as a basis and is compared with observed steady-state crack velocities as a function of film thickness and variations in the curing process. An implication is that on curing, the driving force for film fracture, determined by thermal expansion mismatch, increases less rapidly than the fracture resistance, determined by polymerization.