High sensitivity in-plane strain measurement using a laser scanning technique

Unevenly distributed thermal strain, among materials with different coefficients of thermal expansion (CTE) within electronic packages under operation, affects the working performance for the whole unit. The points that experience the highest strain are most likely to be the failure locations. Knowledge of the strain distribution across the package is in great need in order to analyze the failure mechanism and consequently to improve the design of the packaging. The well-known strain mapping techniques presently in use include Moiré and digital image correlation (DIC) techniques. The former is featured as a full-field and highly sensitive strain sensing technique but is inadequate for spatial resolution when the strain variation of the interested area is very small. The latter is capable of achieving high strain sensitivity and high spatial resolution, but compromises the field of view. This work is to develop a strain sensing technique with high strain sensitivity and high spatial resolution while simultaneously achieving a large field of view. High strain sensitivity is validated by comparing the measured CTE values with reference values from homogenous materials. The system currently in use is capable of making measurements at the 10-micro strain scale which was proven by accurately measuring the CTE of Si. The spatial resolution has been studied by performing a one-dimensional scan across the pre-defined patterns with specific feature sizes. The designed SU-8/Si pattern is to model electronic packages with a simplified structure. Further studies will be continued on electronic packages.