Pan Pacific Symposium Conference Proceedings


Author: Janet E. Semmens et al.
Company: Sonoscan, Inc.
Date Published: 2/10/2004   Conference: Pan Pacific Symposium

Abstract: Acoustic Micro Imaging (AMI) has become an established method to evaluate the quality and reliability of IC packages from the standpoint of package integrity. Ultrasound is sensitive to variations in the elastic properties of materials and is particularly sensitive to locating air gaps (delaminations, cracks and voids). Microscopic feature sizes in flip chip, SOI, and MEMS devices has necessitated imaging at much higher ultrasonic frequencies to improve the axial and spatial resolution.

However, sufficient penetration and working distance must be maintained to access the various internal layers of the devices. Higher frequency pulse - echo transducers have recently been developed and are in use. However, factors such as fluid path attenuation and fluid couplant temperature need to be addressed to realize the resolution improvement. The assumption of higher resolution in the images based on higher frequency alone comes from basic theory and the evidence for the improvements has been based on the clarity of details observed in the acoustic images.

But no accepted resolution test sample for high frequency resolution tests of ultrasonic microscopes/transducers has been available to provide a quantitative, objective measure of the gains in resolution. Such a sample is important for both determining the minimum flaw or feature size detectable in a device using AMI at a given frequency and for establishing if the acoustic microscope and/or transducer are operating to specified parameters.

This paper will describe the resolution test sample and show the results obtained with high frequency transducers. In addition, example images of actual samples will be shown to illustrate the resolution capabilities of high frequency ultrasound. Too often theoretical calculations state the impossibility of detecting certain defects that are seen by practical applications. The Test Wafer was designed to reduce some complex equations to a solid practical base.

Key words: acoustic micro imaging, resolution test wafer.

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