| Device cracking is one of the major reliability issues. The crack propagation is well studied, but the crack initiation is not. In particular, we are lack of knowledge to characterize the thermally induced crack initiation in a ductile/elastic configuration. Using critical stress intensities calibrated by experimental results, this thesis work successfully demonstrates a model to predict the device crack initiation in a solder-to-GaAs configuration. The thesis consists of three major studies.; The stress intensities of a sharp corner are functions of materials properties, notch angle, and far field loadings. The complicated asymptotic solutions for the angular functions are needed for the calculation. Such a need is eliminated by a new simple method developed from this work. This method calculates the stress intensities by using the finite element analysis and the asymptotic stress singular exponents. The new method is proven to be computationally efficient and accurate.; This thesis work uses the critical stress intensities to predict thermally induced crack initiation in the ductile/elastic configurations. The plastic deformation does affect the stress distribution in a region very close to a corner tip. However, in many cases, this plastic deformation does not change the stress intensity-dominant elastic annulus. For a ductile/elastic configuration, as along as the elastic annulus exists, the stress intensities are expected to characterize the crack initiation well. For a solder-to-GaAs local mismatch problem, the elastic annulus exists for the solder pad diameters ranging from 8 mil to 40 mil under a temperature loading DeltaT = --100°C.; A model based on the stress intensities is notch angle dependent, and it has to be calibrated by experimental results. An experimental procedure is established to create and inspect crack initiations under different configurations and thermal loadings. The experimental results determine the critical stress intensity to be 100.4 MPamum0.41 for the solder-to-GaAs configuration with a notch angle of 66.9°. This critical value is successfully applied to predict the crack initiations occurred or not occurred in 25 conditions. The model and the experimental procedure enhance our knowledge to characterize the device crack initiation under thermal loadings. |
