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 trade-off 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.