The x-ray imaging systems used for failure analysis work in much the same way as those used for medical procedures, albeit at a much lower power level. By using an x-ray source and detector, an analyst can study the internal structure of a device to look for defects in the same way a doctor might study an x-ray to look for fractured bones. Depending on the type of device and the reported failure condition, x-ray imaging may be used to look for many different things. When studying an integrated circuit, for example, the x-ray can easily reveal problems with bond wires or flip-chip bumps, often showing open-circuit or short-circuit conditions and eliminating the need to open the package at all. Indeed, in some cases – for example, in the case of adjacent bond wires touching due to wire sweep during packaging – traditional decapsulation of the device can remove any evidence of the failure altogether!
X-ray imaging can also be useful for failure analysis of printed circuit assemblies. Since most modern circuit boards use multiple layers of conductive traces to route signals from point to point, it is not always possible to visually trace the electrical path between components. Since the x-ray can reveal all layers of a board simultaneously, following a signal and pinpointing a failure site is much more straightforward. Furthermore, some defects that may not be evident on visual inspection, like improper via drilling or component misregistration, can be identified much more readily with x-ray imaging.
Non-destructive testing (NDT) – gathering data about a sample without causing any irreversible harm or change – is one of the most important steps of failure analysis. By allowing an analyst to study the internal machinations of a sample without disturbing its physical integrity, x-ray imaging is an integral part of the NDT process.
Derek Snider is a failure analyst at Insight Analytical Labs, where he has worked since 2004. He is currently an undergraduate student at the University of Colorado, Colorado Springs, where he is pursuing a Bachelors of Science degree in Electrical Engineering.