Tingge Xu

Measurement of thickness and profile of a transparent material using fluorescent stereo microscopy

Full-field thickness measurement for a thin transparent film is of interest for biological, medical, electronic, and packaging materials. It is a challenging task when the film is curvy, delicate and its thickness varies with location. We report herein a method to measure the thickness of a transparent (flat or curved) material and its topography using a stereo fluorescent profilometry technique. In this technique, two different types of fluorescent particles are deposited to both sides of the transparent film. Selected fluorescent excitation and emission are used to allow the observation of each one surface of the film at a time to determine the surface profile using stereo-based digital image correlation techniques. After the surface profiles for both surfaces are determined, subtraction of one surface profile from the other provides accurate thickness distribution of the film. Validation experiments were conducted using transparent films with known thickness. As an application, a measurement on a contact lens was conducted. The technique is appropriate for measurement of the full-field thickness of objects at other scales, such as soft transparent or translucent biofilms, with which thickness can hardly be measured accurately with other techniques.

High-Strain Rate Compressive Behavior of a Clay Under Uniaxial Strain State

Dynamic compressive behavior of a clay obtained from Boulder, Colorado was investigated on a 24.4 m split Hopkinson pressure bar. The as-received clay was first pulverized into powders, which were subsequently dried in oven at 105 °C for 2 weeks, and then sieved to collect powers with dimensions smaller than #50 (0.3 mm) using ASTM E-11 standard sieves. The sorted clay was compressed inside a hardened steel tube, by a tungsten carbide rod at one end in contact with the incident bar, and a brass rod at the other end in contact with the transmission bar. This assembly was subjected to repetitive shaking to consolidate the soil to attain a desired bulk mass density; it is then placed between the incident steel bar and the transmission aluminum bar for dynamic compression. Through measurements of both axial and transverse responses of the cylindrical clay specimens under confinement, both volumetric and deviatoric responses were determined under the 3D stress state involved. Both dry and moist clay specimens were characterized at high strain rates. The effect of moisture content on the compressive behavior was investigated. The compressibility as a function of axial stress was determined.

Interaction of Shock Wave with Granular Materials

The mechanical behavior of granular materials such as beads and sand under blast wave involves complex solid–fluid coupling. In this study, a miniature shock tube is developed to investigate the interaction of shock wave with granular materials. A large-opening metal grid is used to support beads. Pressure is measured by pressure sensors as shock wave propagates. Steel beads are used for the investigation of their interaction with shock wave. A Schlieren photography system is used to observe the flow field. A high-speed camera is used to acquire video of the wave. The interaction of shock wave with beads is observed and discussed.

Fluxless Bonding Process Using Thermo-Compression Micro-Scrub for 61 µm Pitch SnAg Solder 3-D Interconnections

In this research study, we evaluated the use of a micro-scrub bonding process as a potential method to eliminate the use of flux, and its associated cleaning, during flip chip thermo-compression bonding. Additionally, this process requires no plasma pre-treatment to eliminate tin oxide and contaminants from the micro-bump surface. This evaluation used a test vehicle with 20-μm/15-μm thick Cu/SnAg micro-bumps and Ni/Au pads at 61 μm pitch. The top die has more than 170,000 micro-bumps and both the top die and bottom die were larger than 600 mm2. The top die and bottom die were bonded with parameters developed for a micro-scrub bonding process. After each die makes contact with precise alignment, micro-scrub is applied under a specific force, temperature, and atmosphere during the bonding process. The mechanical micro-scrub action breaks up the native tin oxide film and improves the wettability of the solder. X-ray images confirmed there was no solder bump bridging across the full die area. Neither voids nor cracks were observed at the solder joints by SEM cross sectional analysis. CSAM results indicated there were no voids in the underfill.

High-Strain Rate Compressive Behavior of a “Natural Soil” Under Uniaxial Strain State

Dynamic compressive behavior of a synthetic “natural soil” was investigated on an 85-ft split Hopkinson pressure bar. The “natural soil” was prepared by mixing a sorted dry clay and assorted dry Mason sand. The as-received moist clay (obtained from Boulder, Colorado) was first pulverized into powders, which were subsequently dried in oven at 105 °C for 2 days. The as-received Colorado Mason sand was dried under the same conditions. Then the clay was sieved to powers with dimensions smaller than #50 (0.3 mm) using ASTM E-11 standard sieves. The “natural soil” was obtained by mixing at different ratios of clay and sand at 5:100, 10:100, 20:100, 100:100 weight ratios. The mixed soil was capped inside a hardened steel tube, by two tungsten carbide rods at both ends in contact with the bars. This assembly was subjected to repetitive shaking to consolidate the soil to attain a desired bulk mass density; it was then placed between the incident steel bar and the transmission bar for dynamic compression. Through measurements of both axial and transverse responses of the cylindrical soil sample under confinement, both volumetric and deviatoric responses were determined. Both dry and water-saturated soil samples were characterized at high strain rates. The effects of different mixed ratio of clay/sand, and the water-saturation on the compressive behavior were investigated. The compressibility as a function of axial stress was determined. These results are compared with pure clay and sand samples.

Tri-layer nanoindentation for mechanical characterization of ultra-low-k dielectrics

Recently the mechanical properties of nano-porous ultra-low-k (ULK) dielectric thin films have been characterized by nanoindentation using a tri-layer sample configuration. Tetraethyl orthosilicate (TEOS) silica was coated on the fragile ULK thin film to protect it from direct contact with nanoindenter tip. In this paper, the finite element method (FEM) simulations are conducted to investigate the effect of TEOS thickness, and to propose rules to design the tri-layer sample.