Applications of Advanced Imaging Methods : Macro - scale Studies of Woven Composites and Micro - scale Measurements on Heated IC Packages

As a representative advanced imaging technique, the digital image correlation (DIC) method has been well established and widely used for deformation measurements in experimental mechanics. This methodology, both 2D and 3D, provides qualitative and quantitative information regarding the specimen’s non-uniform deformation response. Its full-field capabilities and non-contacting approach are especially advantageous when applied to heterogeneous material systems such as fiber-reinforced composites and integrated chip (IC) packages. To increase understanding of damage evolution in advanced composite material systems, a series of large deflection bending-compression experiments and model predictions have been performed for a woven glass-epoxy composite material system. Stereo digital image correlation has been integrated with a compression-bending mechanical loading system to simultaneously quantify full-field deformations along the length of the specimen. Specifically, the integrated system is employed to experimentally study the highly non-uniform full-field strain fields on both compression and tension surfaces of the heterogeneous specimen undergoing compression-bending loading. Theoretical developments employing both small and large deformation models are performed. Results show (a) that the Euler–Bernoulli beam theory for small deformations is adequate to describe the shape and deformations when the axial and transverse displacement are quite small, (b) that a modified Drucker’s equation effectively extends the theoretical predictions to the large deformation region, providing an accurate estimate for the buckling load, the post-buckling axial load-axial displacement response of the specimen and the axial strain along the beam centerline, even in the presence of observed anticlastic (double) specimen curvature near mid-length for all fiber angles (that is not modeled), (c) for the first time show that the quantities σeff εeff are linearly related on both the compression and tension surfaces of a beam-compression specimen in the range 0 ≤ εeff < 0.005 as the specimen undergoes combined bending-compression loading. In addition, computational studies also show the consistency with the experimental σeff εeff results on both surfaces. In a separate set of studies, SEM-based imaging at high magnification is used with 2D-DIC to measure thermal deformations at the nano-scale on cross-sections of IC package to improve understanding of the highly heterogeneous nature of the deformations in IC chips. Full-field thermal deformation experiments on different materials within an IC chip cross-section have been successfully obtained for areas from 50x50 μm to 10x10 μm and at temperatures from RT to ≈ 200C using images obtained with a Zeiss Ultraplus Thermal Field Emission SEM. Initially, polishing methods for heterogeneous electronic packages containing silicon, Cu bump, WPR layer, substrate and FLI (First level interconnect) were evaluated with the goal of achieving sub-micron surface flatness. Studies have shown that surface flatness of 700nm is achievable, though this level is unacceptable when using e-beam photolithography for nanoscale patterning. Fortunately, a novel self-assembly technique was identified and used to obtain a dense, randomly isotropic, high contrast pattern over the surface of the entire heterogeneous region on an IC package for SEM imaging and DIC. Experiments performed on baseline materials for temperatures in the range 25°C to 200°C demonstrates that the complete process is effective for quantifying the thermal coefficient of expansion for nickel, aluminum and brass. The experiments on IC cross-sections were performed when viewing 25μm x 25μm areas and correcting image distortions using software developed at USC. The results clearly show the heterogeneous nature of the specimen surface and non-uniform strain field across the complex material constituents for temperatures ranging from RT to 200°C. Experimental results confirm that the method is capable of measuring local thermal expansion in selected regions, improving our understanding of these heterogeneous material systems under controlled thermal-environmental conditions.