Electronic devices and packages composed of complex material systems undergo unevenly distributed thermo-mechanical strain during their life cycle because of the mismatch of coefficients of thermal expansion (CTE) among the silicon and different packaging materials, which affects the working performance and reliability of the electronic products. Following the accelerated miniaturization trend, microelectronic packages have been pushed forward to be smaller and smaller in order to meet the demands of the market. The inevitable consequences of miniaturization include increased heat dissipation among layers of different packaging materials, which results in various degrees of deformations in the layers due to a CTE mismatch. The deformation brings in non-uniform distributions of strain across the whole package, where spots experiencing the highest strain are most likely to fail. A localized strain distribution map, coupled with an understanding of the material properties, could be a good indication of the potential reliability failure locations in the package. There is a great need for knowledge of the strain distribution across the package in order to analyze the failure mechanism and consequently to improve the design of the packaging.
The often used strain sensing techniques capable of strain mapping include micro Moiré, optical digital image correlation (DIC), and scanning electron microscope (SEM) DIC, etc. Micro Moiré has been proven to be a highly sensitive, full-field in-plane sensing technique. However, the illuminated area for generating a Moiré pattern needs to be large enough to detect small strains; consequently, it lacks the ability to resolve strains with small spatial variations. DIC techniques can achieve a high spatial resolution with high in-plane displacement resolution. However, the field of view is compromised since a large optical magnification is required, and becomes a limiting factor when detailed strain mapping in a large area is needed.
Therefore, there remains a need for a strain sensing technique to map the in-plane strain distribution with high strain sensitivity, high spatial resolution, and a large field of view.
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