Transient thermal strain measurements in electronic packages

Fractional fringe moire interferometry was successfully applied to monitor thermo/mechanical strain histories in a memory device as the temperature was changing from room temperature (25 degrees C) to an arbitrarily chosen final temperature of 80 degrees C. Full displacement fields were recorded at different temperatures and used to predict strains in a small region at the chip corner. Moreover, strains across the chip thickness along a line close to the chip corner were investigated. A dramatic change in the pattern of axial strain growth was observed for temperatures around 70 degrees C and above. It is suggested that the development of severe compressive stains due to bending of the package is responsible for such a change. A minor deficiency in the data collection procedure affected the accuracy of strains predicted for the selected region. This necessitated the use of a least-squares curve fitting procedure to represent deformations based on experimentally collected values. This yielded smooth strain distributions giving a clear qualitative picture of strain histories, but is introduced some errors in the strain magnitudes (10-15% of actual values).<
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