Packaging Induced Die Stresses—Effect of Chip Anisotropy and Time-Dependent Behavior of a Molding Compound

This paper investigates the effect of the anisotropic behavior of the die and the time- and temperature-dependent behavior of epoxy molding compound on the packaging induced stresses for a quad flat package. Finite element (FE) simulations using isotropic and anisotropic properties of the die are carried out, respectively, and the results are compared. Creep experiments were performed at different temperatures ranging from 265°C to 230°C to obtain the long-term master curves and the related shift factors for the creep compliance of the molding compound. FE models which incorporate the viscoelastic constitutive relation of the material are constructed to simulate the thermo-mechanical stresses caused by the packaging processes. The influences of both the chip anisotropy and the viscoelastic behavior of the molding compound on the packaging induced stresses are discussed. @DOI: 10.1115/1.1604153# At present, thermo-mechanical reliability of integrated circuit ~IC! packages is still one of the major concerns in the electronic industry. Critical stress levels may be induced in the package constituents during the thermal processing due to mainly the mismatch in thermal-expansion coefficients of the materials. The prediction of these packaging induced thermo-mechanical stress levels can only be true if reliable material properties for each constituent are taken into account in the modeling. In most of the publications addressing the thermo-mechanical behavior of IC packages, the silicon die was modeled as temperature independent and isotropic. However, given the nature of the silicon material, which is a crystal, the assumption of isotropy is not true. In fact, due to the orientation of the silicon crystal, a diamondlike crystallographic structure, anisotropic material behavior is to be expected. Ultrasound measurements ~20/40 MHz! were used to obtain the stiffness values in the different directions for the silicon crystal @1‐3#. It was reported that the stiffness values at temperature 273 K range from 170 GPa in the @110# plane to 130 GPa in the @100# plane @2#. These values show that an isotropic approach of the silicon material may not be valid. The temperature dependence of the stiffness values was found to be negligible, for instance the in plane value at 473 K is only 0.5% lower. Previous research work also showed that the temperature independence of the thermal-expansion coefficient ~CTE! for the silicon crystal is valid within a certain temperature range @3‐6 #. Thermosetting resins, like other polymeric materials, have strong time- and temperature-dependent mechanical properties even if they are filled with a high percentage of filler. The creep and relaxation of the packaging material during packaging processes and/or testing will cause a redistribution of stress and strain levels in the chip. However, for the reason of simplicity, in most of the thermo-mechanical packaging simulations the viscoelasticity of the molding compound is totally or partially neglected. As a consequence, the predicted stress levels and its evolution during packaging processes and/or testing may not be representative for the reality. In Refs. @7‐9# it was reported that by using a viscoelastic model for the molding compound, the predicted stresses and deformations are closer to the real situations. The present study focuses on the investigation of the influence of chip anisotropy and the time- and temperature-dependent behavior of a molding compound on the stress levels during packaging processes and testing conditions. In the finite element ~FE! simulations, the anisotropic and isotropic properties of the silicon die are used, respectively, and a comparison of their results is made. Creep experiments were performed at different temperatures ranging from 265°C to 230°C to obtain the long-term master curves and the related shift factors for the creep compliance of the molding compound. FEM models, representing a quad flat package ~QFP! package, in which the viscoelastic model of the material is implemented, are constructed and used to simulate the thermo-mechanical stresses caused by the packaging processes.