Creep behavior of a molding compound and its effect on packaging process stresses

Critically high thermal stresses are induced in the constituents of an electronic package during packaging processes, due to the mismatch of the thermal expansion/contraction of the constituents. These stresses may cause cracks in the silicon die in some package configurations. The temperature-dependent creep behavior of an epoxy molding (packaging) compound is studied here in order to analyze the stresses induced in the packaging processes reliably. Isothermal one-day creep experiments are performed at different temperatures ranging from 24.5°C to 175°C (above the glass transition temperature of the compound). Significant creep behavior of the epoxy compound is observed even at room temperature. The tensile creep compliance and the increasing time-dependent Poisson´s ratio of the material at different temperatures are successfully used to construct viscoelastic master curves for these material properties. It is observed that the shift factor of the compound cannot be fitted by the well-known WLF equation. Further, the viscoelastic model of the material is implemented in a finite element program and verified by means of the results of a creep test that is performed at a non-isothermal condition. Moreover, the effect of the creep behavior of the molding compound on the packaging process stress field and its evolution is investigated. Finally substantial cost saving is realized by package design optimization based on the reliable prediction of the packaging process stresses