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Failure analysis using Liquid crystal Imaging - Understanding the procedure

Failure Analysis using Liquid Crystal ImagingIntegrated circuits are getting more complicated all the time, necessitating a need for a wide variety of analysis techniques to figure out what's wrong. When so many electronic components are crammed together, the chances of one of them failing is very high. Detecting them is anything but trivial given the fact that the high density makes it challenging to isolate the fault. And this is where failure analysis using liquid crystal imaging comes into play.

Misbehaving components create what we call "hot spots" on the chip. As the name implies, they exhibit a higher temperature than the surrounding areas. It's not easy to detect them since the temperature difference is too low to measure using conventional means. In this article, we see how liquid crystals are used to do the job.

Using Liquid Crystals

Most liquids are isotropic, meaning that there's no order to their molecules. They align themselves randomly and there's no structure to them. Crystals of course are unique for their orderly arrangements. Liquid crystals however display order in their molecules while still maintaining a liquid form. We exploit their characteristics for micro thermography - the field of detecting tiny variations in temperature.

Thermotropic liquid crystals are those whose molecules respond to changes in temperature. Too high a temperature and they lose all semblance of order given the high energy that excites them. Too low a temperature and they can easily solidify into crystalline form. In the correct temperature range however, the molecules of the liquid crystal change in easily observable ways and this is what we make use of.

We start by applying a fine layer of the liquid crystals onto the die. After this, the circuit is fired up by passing current through it. Based on the response of the liquid in various places, we can make out the spots that generate excessive heat which ultimately guides us to isolating the problem. Depending on the sensitivity of the setup, we can detect differences in currents which are as low as 10 µW.

Of course, interpreting the data is another matter. As any javascript or PHP programmer knows, the specific line of code which throws the error need not be the line on which the problem is located. Similarly, the mere presence of heat isn't a sure shot indicator of a fault. Even property working circuits can be coerced into generating abnormal levels of heat by a malfunctioning one down the line. This requires proper expertise.

Failure analysis using liquid crystal imaging is a non destructive process - making it valuable with expensive circuits. When combined with other methods of analysis, we get a comprehensive failure detection system which catches any type of error in the integrated circuit.

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