During normal power cycling of the electronic equipment, the differing coefficients of thermal expansion result in differential elongations. Because each level of packaging is subject to mounting constraints, the differential strains result in bending and shear stresses. Repeated duty cycling can cause fatigue at joints, at interfaces between different materials, at interconnection locations, or cause delamination of composite materials. Accelerated Thermal Cycling (ATC) is done to simulate the fatigue failures that may arise because of this power cycling. The current practice is to determine ATC stresses by assuming that the temperatures of various layers are equal and constant. In this study, we have relaxed the isothermal assumption and we provide results for thermal stresses and strains in a first level package. This is accomplished by accurately determining the transient temperature fields in various layers of the package. Temperature variations for different heat transfer coefficients have also been calculated. The results indicate that realistic estimates of thermal stresses and strains are only possible with models that allow for temperature variation in the body of the package. High equivalent stress values are obtained at the chip-heat sink interface and in the bumps connecting the leads to the chip.

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