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Troubleshooting, failure analysis for ICs

07 Dec 2012  | Bill Laumeister

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This approach, admittedly, conflicts with those who believe that an FA should be performed with no delay. We heard, "that an FA is always the first thing to be done. Looking at the internal parts of the IC should be done before looking at the IC in the application circuit." We do not understand where that idea originated, and we disagree. The FA is not the first task. Rather, investigating the "scene of the crime," the failure incident, is the first step.

The information at the failure location is critical and, like police investigators, we should go to extreme lengths to preserve the on-scene data. The first thing is to investigate the IC in the application circuit, i.e., where it failed. A simple thing like a solder splash may be the key to the answer. The IC might be partially operational but not totally failed. In fact, removing the IC could mask the real problem.

For an effective FA we need to check a customer's schematic diagram and gather all the circumstances, the reasons, for the failure. Yes, this procedure may well confront a customer's confidentiality issues. This is a common concern, which is why there are nondisclosure agreements (NDAs).

This is also a situation where FAEs serve as the factory's eyes and ears on the ground all over the world. FAEs can go into the customer's facility and evaluate the schematics, layout, and other conditions for the application. To protect customer confidentiality, the FAE need only send QA the relevant parts of a customer's design schematic. And now, finally, QA will be working with credible failure data.

A successful outcome
Back to our story. We can say that the local FAE did get involved more closely with the customer on this failure issue. With more schematics in hand, here is what little we saw. An op amp connects to an output pin, but it should have little effect because of the 10kΩ series resistor.

By using one common ground, not separate grounds connected at one star point, noise on one supply is directly coupled though the decoupling capacitors to other supplies. The smallest decoupling capacitors are 0.1µF. Typical surface mount 0.1µF capacitors are self-resonant at about 15MHz; above that frequency they are inductors and cease to function as capacitors.

There are two lessons from this. First, decoupling capacitors are a two-way street. If one couples a noisy power supply to a quiet supply, the noise will contaminate the quiet supply. Second, the same thing happens with a noisy ground: the noise will contaminate the quiet supply. Noisy supplies need to be paired with a noisy ground and clean or quiet power must be paired with clean power. Cross-contamination can hurt both powers and grounds. Above the capacitors' self-resonance frequency, it becomes inductive, that is, it does not conduct or attenuate high-frequency energy.

Conclusion
So we come full circle and repeat an opening comment: knowledge is king when troubleshooting an IC failure. From the outset of an investigation no one is more valuable than the local FAE who examines the issues side by side with the customer. The FAE must scrutinise the whole system, board layout, schematics, and application, and then convey that data back to QA. Only with accurate, detailed incident data can we solve IC failure issues. Without that data, QA is forced to guess about the "scene of the crime."

About the author
Bill Laumeister is an engineer in strategic applications with the Precision Control Group at Maxim Integrated Products. He works with customers who use DACs, digital potentiometers, and voltage references. He has more than 30 years of experience and holds several patents.

To download the PDF version of this article, click here.


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