Evaluation of coherent fibre-bundle characteristics for use in an ESPI-based instrument for measuring strain in electronic packages

摘要

Modem connection and packaging methods used in integrated circuit manufacture, in particular ball-grid array (BGA) and flip-chip techniques, involve the compression at elevated temperatures of solder balls and gold bumps between the die of silicon or GaAs and a substrate typically of alumina, glass-epoxy composite or AIN. The die and substrate generally have widely differing coefficients of thermal expansion, which results in residual stress in the assembly on cooling, and subjects the solder balls or gold bumps to shearing forces. A prototype instrument based on electronic speckle pattern interferometry (ESPI), currently under development at EMPA as part of the HIRONDELLE project, is intended to measure the strain fields developed in BGAs and other devices under thermal load. One mode of operation of the instrument involves the use of coherent fibre bundles to transfer the images of the BGA to the object plane of the camera. We report here on the tests we have performed on different types of bundles to measure optical parameters relevant to our micro-imaging application: cross-talk between adjacent fibres, optical transmission quality; modulation transfer function and limiting resolution; and fringe density and modulation in an ESPI set-up. On the basis of the results we were able to select the bundle most suitable for the HIRONDELLE instrument.