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| Jun
Tian |
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The
following are abstracts based on work I have conducted in our lab.
The last topic regarding reliability measurements on microelctronic
packages is what my current dissertation work involves. |
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| Title: A failure criterion of graphite/epoxy laminates under In-plane uniaxial and biaxial compression |
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Failure mechanisms are investigated for AS4/3502 and IM7/8551-7 carbon/epoxy cross-ply laminates under both uniaxial and biaxial compression. The overall failure modes transited from in-plane shear to delamination with the increasing of loading in the secondary direction. The stresses in the laminates upon peak loading are calculated using CLT method. By a careful examination on the failure zone under SEM and an investigation on the stress state, the microscopic failure mechanism of these samples is determined to be either matrix shear failure or fiber shear failure. Fiber microbuckling is also observed in one of these samples. The computational results reveal that, for both the fibers and the matrix, the shear failure strength is linearly dependent on the normal pressure on the shear plane. Based on this fact, a local failure criterion is proposed which is in a similar form to the Coulomb-Mohr criterion. This local failure criterion is then extended to the laminates level to meet the design purpose. This mechanism-based overall failure criterion suggests that failure occurs when the maximum shear stress in the laminates reaches its linear pressure-dependent shear strength. |
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Title:
Glass-modified stress waves for adhesion measurement of ultra thin
films for device applications |
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Laser-generated stress wave profiles with rarefaction shocks (almost zero post-peak decay times) have been uncovered in different types of glasses (see image below). The formation of the rarefaction shock is attributed to the increased compressibility of glasses with increasing pressures. This was demonstrated using a one-dimensional nonlinear elastic wave propagation model in which the wave speed was taken as a function of particle velocity. The technological importance of these pulses in measuring the tensile strength of very thin film interfaces is demonstrated by using a previously developed laser spallation experiment in which a laser-generated compressive stress pulse in the substrate reflects into a tensile wave from the free surface of the film and pries off its interface at a threshold amplitude. Because of the rarefaction shock, glass-modified waves allow generation of substantially higher interfacial tensile stress amplitudes compared with those with finite post-peak decay profiles. Thus, for the first time, tensile strength of very strong and ultra thin film interfaces can be measured. Results presented here indicate that interfaces with strengths as high as 2.62 GPa and adhesion of films with thickness as low as 320 nm can be measured. Thus, an important advance has been made that should allow material optimization of ultra-thin layer systems that may form basis of future MEMS-based microelectronic, mechanical and clinical devices.
*An acrobat copy of this journal paper can be download by clicking here. |
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| Title: Reliability measurement of microelectronic packages | ||||
Various kinds of testing methods such as direct pull tests, shear tests, drop tests etc. have been employed in the industries for the reliability measurement of microelectronic packages. In the research work presented here, the basic laser spallation method, which was originally developed to measure interface strengths of planar film systems, is used to measure the tensile strengths at interfaces within flip-chip packages for the first time (see image below). Unlike the simple thin film systems, the flip-chip packages have complex geometries and material compositions. Therefore a special three-step procedure including a finite element model is used to measure the interface strengths. Contrasted to the methods used in the industry, this technique not only saves time (causing interface delamination within several nanoseconds) and costs, but also allows a fundamental understanding of the adhesion between different layers as a function of process and service variables such as moisture and temperature by a correlation between the interface tensile strength and the interface microstructure.
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