Acoustic Microscopy - A Sounding Board for Defects

The most common type of defect found with acoustic microscopy is package delamination. In a typical plastic encapsulated integrated circuit, the mold compound adheres perfectly to the surface of the die; however, physical stress or contamination can cause the mold compound to pull away from the die, leaving a gap between the two materials. This gap can spell disaster for the part; if the air is trapped in the space between the die and the mold compound, differences in thermal expansion behaviors between the two materials can cause the device to slowly tear itself apart under temperature cycling, breaking bond wires and causing open circuit conditions. Similarly, delamination that has trapped moisture between the die and mold compound can result in "popcorn cracking"; when the device is soldered onto a printed circuit board, the trapped moisture will vaporize, creating internal pressures that eventually rupture the package, just like a kernel of popcorn in the microwave. Acoustic microscopy is very sensitive to package delaminations, as the acoustic impedance mismatch between the plastic encapsulant material and the trapped air or moisture has a profound effect on the reflected sound wave the system transforms into an image. By scanning the entire device, an analyst can quickly and efficiently create an image showing the location of any and all delamination on the device; these results may provide the necessary clue to identify the root cause of failure on a malfunctioning part.

The acoustic microscope can also be used to look for voids in solder or other types of die attach materials. Many semiconductor devices depend on their die attach for proper heat dissipation; if the material has large voids in it, it will not conduct heat properly and the device will overheat, greatly reducing its viable lifespan and in some cases leading to catastrophic failure. The ultrasonic waves generated by the acoustic microscope can easily penetrate a device to detect any solder or die attach voiding. This is useful not only for failure analysis, but also for reliability studies; parts that have been subjected to environmental or electrical stresses as part of a characterization study can be analyzed with the acoustic microscope, and any voids or other anomalies can be recorded. Studying this data can allow a manufacturer to predict the useful longevity of their devices and plan accordingly, making changes in device construction and processing if necessary to meet reliability goals.

The inclusion of acoustic microscopy in a failure analyst's toolkit greatly expands their capabilities, providing a non-destructive method for analyzing many types of devices. With the data generated by an acoustic microscope, an analyst can provide valuable information to their customer or elect to perform further analysis, characterizing the part in even more detail to give the manufacturer a crystal-clear picture of the root cause of failure for a defective unit.

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