Interface fracture strength is evaluated by the Weibull stress criterion for LSI devices composed of epoxy resin and Fe-Ni alloy sheet. The difference in coefficient of thermal expansion of the epoxy resin and Fe-Ni sheet causes a stress singularity at the corner ends of the device in the cooling process during LSI packaging, which is responsible for the interface fracture of the device. It is shown that the critical Weibull stress, an integrated stress over the stressed body along the interface, at fracture initiation is almost independent of the package geometry, although both the local stresses and the singularity parameters in the LSI device significantly depend on it. Shape design to decrease the Weibull stress values would be effective to improve the LSI devices reliability.