Current Issue The International Reference for Chip-Scale Electronics, Flip-Chip Technology, Optoelectronic Interconnection and Wafer-Level Packaging November - December 2001 Email the editor

By Steve Martell, Sonoscan Inc., Elk Grove Village, Illinois

Because CSPs pack great power into very small areas, the hidden internal defects that wreak havoc with CSPs are likewise very small.

Accurately imaging and correctly diagnosing internal defects require the very high resolution of acoustic micro-imaging.

Image resolution is usually thought to be synonymous with the frequency of the ultrasound pulsed by a given transducer. The higher the frequency, the greater the spatial resolution. In some limited cases, this statement is basically true.

Very high resolution image gated at the second layer of metallization in a CSP. Irregular dark areas are voids in the elastomer layer.

Usually, however, there are other critical factors whose overall impact is such that a transducer with a higher frequency may offer less spatial resolution than a transducer of lower frequency.

The design of the transducer lens, the frequency spectrum of the pulser and the thickness of the sample (especially at very high frequencies) matter greatly.

Different transducers with the same nominal frequency can be designed to do different jobs. Therefore, you may find a group or family of transducers with the same or similar nominal frequencies, which are designed for different purposes.

Optimum results in imaging a given sample take place when the right transducer is used on the sample.

The number of data points collected during scanning also affects resolution in the acoustic image.

For years the standard was 512 x 480 pixels. Where truly superb resolution is needed-as in CSPs-the new high-end image size of approximately 8,000 x 8,000 pixels is a good choice. [sonoscan.com]