206 K. B. Connolly and W. C. Ralph DIGITAL MICROSCOPE INSULATION W/AIR VENTS LOW THERMAL EXPANSION STAGE RESISTANCE HEATER SPECIMEN Fig. 27.3 Schematic of prototype microscopy apparatus ceramic bearings allowing for greater airflow under the stage, further disrupting heat distortions and keeping the microscope lens cooler. A translation stand was added under the insulation layer to allow for precision control of specimen placement. Finally, the lens was replaced with a telecentric lens to improve the depth of field that will remain in focus. 27.3 Specimen Preparation Preparation of the specimen surface is required to produce good results. DIC requires high-contrast surface features in order to track the displacement in the images. These features need to be a certain number of pixels in size and remain consistently identifiable throughout the test. On rare occasions, the surface to be measured has sufficient contrast that it can be measured as-is. An example of this is shown in Fig. 27.4, in which an as-manufactured second level interconnect copper pad from a microelectronics package is photographed at 1000×magnification and shows high contrast surface features. It is more often the case that the specimen must be prepared in order to add or reveal high contrast features. Typical specimen preparation in centimeter scale specimens is typically done using spray paint to produce a field of randomly shaped and sized speckles, and a variety of techniques can be used to produce acceptable sizes of speckles. Paint droplets are too large for millimeter scale specimens, even when applied with an air brush, and particles can be adhered to the surface instead. For objects that are approximately 3 mm or less in width, powder particles of adequate size (less than or equal to 10μm) tend to clump together due to van der Waals forces and result in features that are too large for the scale of the image and the depth of focus of the lens. Two techniques have been used successfully to create contrast in microscopy specimens. The first is to finely polish the specimen surface to reveal the microstructure of the materials. The microstructure features may include the grain or dendritic structures of metals, such as solder; small inclusions or voids in metals, silicon, or polymers; filament ends or edges in composites; or any other feature that produces sufficient contrast. An example of this is shown in Fig. 27.5, where a pair of first-level interconnect solder bumps has been polished to reveal the solder dendrites, voids in the copper traces and silicon, along with features in the polymeric underfill and substrate resin. All of these features produced sufficient contrast for DIC software to track. At lower magnifications, or on materials with insufficient intrinsic features to produce surface contrast, the specimen surface can be less finely polished to produce surface features. An alternative technique is to finely polish the surface and then “de-polish” the surface with a coarser compound or paper. An example of this is shown in Fig. 27.6, where small scratches produced contrast in regions of the solder microstructure with insufficient intrinsic features. This technique tends to produce higher levels of signal noise in the data, which may be due to the depth or relatively uniform nature of the marks. Additional techniques for creating contrast are still being explored to create smaller trackable features at higher magnification. Most of these techniques involve powder mixes being applied to the specimen surface. Concern still exist to make sure that contrast material stays adhered to an inverted surface. The inverse of this problem also exists if the surface coating is too thick it will effect strain measurements.
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