One of the most critical points of any failure analysis is the decapsulation step. Decapsulation is the point where non-destructive analysis ends and more risky operations begin – the die is removed from its protective plastic shell to allow failure analyst access to the complex circuitry within. Usually, decapsulation is performed using wet-etch procedures, dissolving the plastic encapsulant material of an IC package with any of a variety of different acids or solvents. The downside of this approach, of course, is that working with these potentially hazardous chemicals necessitates some serious safety measures like fume hoods and other types of personal protective equipment. Furthermore, the chemicals most often used for decapsulation, though relatively common, are still not cheap and can amount to a significant expense depending on the number and type of parts that must be decapsulated. Most importantly, the chemical decapsulation process can often disrupt the failure on the part; in some cases, like when working with GaAs or some other III-V semiconductors, the decapsulation chemicals can even dissolve the integrated circuit completely! Fortunately, there is an experimental alternative to chemical decapsulation: laser decapsulation is one of the most promising new technologies on the horizon.
Laser decapsulation works by using a high-power laser to ablate the plastic encapsulant material away from a device. By vaporizing the encapsulant material instead of dissolving it, the risk of accidentally destroying or washing away a defect with decapsulation chemicals is eliminated. Furthermore, since the laser decapsulation system is controlled with precision stepper motors, it is possible to decapsulate devices that would be exceptionally challenging to expose chemically; small or oddly-shaped devices can be attacked with a laser with the same degree of ease as a more standard device. Another benefit of laser decapsulation is a reduced need for chemical consumables.
Laser decapsulation is still in its infancy, however, and still possesses several major flaws; though there is a reduced need for chemical consumables, the decapsulation process will still involve a wet-etch step, since allowing the laser to drill all the way down to the die would invariably damage the sensitive circuitry there. Moreover, the laser can cause damage to bond wires and, if improperly configured, create a pyrolized mold compound that is nearly impossible to remove, chemically or otherwise.
At present, laser decapsulation is really only suitable as a “pre-decap” step, preparing a sample for chemical decapsulation. This can still be considered an improvement, as it does limit the number of consumables used and the exposure of the device to chemicals. It is almost a certainty, however, that this technology will continue to improve – perhaps eventually getting to the point that wet etch decapsulation can be relegated to the annals of failure analysis history.