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It is common to apply scanning acoustic tomography (SAT) for microelectronic packages inspection because it enables nondestructive imaging of defects. SAT phase inversion imaging is useful tool for detecting defect areas accurately and quickly. Phase inversion imaging is based on the phenomenon of the phase inversion of an ultrasound wave at interfaces in a sample. We propose a signal processing method for phase inversion imaging. Specifically, it is a robust method of finding the phase inversion. With this method, the phase inversion is found by estimating the correlation between a reflected wave signal of a sample and reference wave signal. A negative correlation between these signals indicates the existence of defects in the sample. Defect detection tests with the proposed method were carried out using integrated circuit packages. The tests results indicate that the proposed method improves robustness for detecting defects compared with a conventional method. The tests also demonstrated that small defects on the order of 100 micrometers could be detected using an ultrasound probe with a nominal frequency of 75 MHz.

This paper describes an inspection technique for detecting bonding defects in interfaces of stacked wafers that have complicated patterns. This technique detects the minute defects as statistical outliers from scanning acoustic tomography images that include the pattern formed repeatedly. The experimental results show that one-pixel size defects near the complicated patterns can be detected from ultrasound images.

This paper describes an image restoration technique that improves the sharpness of an inspection image for LSI packages using a scanning acoustic tomograph(SAT). This technique performs optimal restoration, depending on the type of probe used for imaging, by estimating the intensity distribution of the ultrasonic beam at the interface of testing and by eliminating the influence. The result of an experiment shows that the contrast of SAT images is improved 1.6 times.

We have newly developed a scanning acoustic tomograph with higher resolution and detectability than the conventional type. The system can be equipped with a special signal processing unit that has been designed in order to emphasize the high frequency component of ultrasound. The signal processing unit enhanced the resolution and signal to noise ratio. Because of improvement in the signal to noise ratio, a slit with a width of 1 μm was detected by the new system with the signal processing unit.