ADVANCEMENT OF MOIRÉ INTERFEROMETRY FOR RATE-DEPENDENT MATERIAL BEHAVIOR AND MICROMECHANICAL DEFORMATIONS

Abstract

Moiré interferometry is an optical technique to map full field in-plane deformations with extremely high resolution and signal to noise ratio. The technique is advanced and implemented to study the rate-dependent thermo-mechanical behavior of Sn-based Pb-free solder alloys and micromechanical deformations. In Part I, the mechanical/optical configuration of moiré interferometry for real-time observation of thermal deformations is enhanced to provide measurement capabilities required for the analyses. Two most notable advancements are (1) development of a conduction-based thermal chamber for a wide range of ramp rates with accurate temperature control, and (2) implementation of microscope objectives in the imaging system to observe a microscopic field of view. The advanced system is implemented to analyze the anisotropic behavior of Sn-based Pb-free solder alloys. A novel copper-steel specimen frame is developed to apply a controlled loading to single-grain solder joints. After measuring the grain orientation by electron backscatter diffraction (EBSD), detailed in-situ deformation evolutions and accumulated deformations of solder alloys are documented during a thermal cycle of -40 °C to 125 °C. The results quantify grain orientation-dependent deformations that can lead more accurate anisotropic constitutive properties of Sn-based Pb-free solder alloys. In Part II, an advanced immersion microscopic moiré interferometry system based on an achromatic configuration is developed and implemented for higher displacement sensitivity and spatial resolution. In order to achieve the desired displacement resolution, a high frequency grating (2500 lines/mm) is fabricated on a silicon substrate using lithography first. The square profile is subsequently modified by reactive-ion etching so that it can be used to produce a specimen grating by replication. Secondly, the algorithm of the optical/digital fringe multiplication method is improved to further enhance the measurement resolution of the immersion microscopic moiré interferometry. The system and the noise-free grating are used to analyze thermal deformations of micro-solder bumps. With the basic contour interval of 200 nm, the displacement resolution of 25 nm is achieved with the multiplication factor of 8.