How can one determine the root cause of an ESD issue?

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Tracing the Root Cause of an ESD Problem

Determining the root cause of a recurring or lingering ESD problem is one of the most difficult challenges faced by semiconductor manufacturing and electronic assembly companies today. Indeed, it is better to design your ESD protection system well right from the start to minimize ESD issues than to contend with them on a regular basis later on.

The culprit behind any ESD issue, of course, is the fact that a static charge build-up has occurred somewhere in the line, and was allowed to dissipate in an uncontrolled manner. Thus, most ESD experts' usual advice is to prevent static charge build-up everywhere in the production area - something that isn't easy to do given that transfer of electrons from one body to another is a natural phenomenon.

If you have a recurring and lingering ESD problem, then you know that you are allowing static charge to build up somewhere in your line, and the accumulated static charge is somehow being discharged through your units. Unfortunately, tracing where these static build-ups occur can be very difficult due to two major reasons: 1) static build-up can occur anywhere and anytime, if the conditions are right; and 2) static charges can disappear instantaneously (by discharging through a path made temporarily available to them) even before you find them, leaving no trace that they were there before. It is, therefore, very difficult to spot an ESD event as it occurs.

Nonetheless, there are some steps that one can follow to increase the chances of resolving an ESD issue. The first step in addressing an ESD problem is to have a firm grasp of whether your ESD issue at hand is due to a distinctly new mechanism of ESD, or it is simply a recurrence of a previously encountered ESD event. A good FA of your ESD failures is required to establish this. The analyst, through a meticulous inspection of the failure attributes exhibited by the dead bodies, should be able to say if you're dealing with a new ESD mechanism or a recurrence of an old issue. The nature of the ESD event, including its damage path, must likewise be established by FA.

A recurrence of an old ESD issue would beg answers to the questions: 1) did we implement the right corrective/preventive actions when we first encountered this issue?; 2) are these actions sufficient enough to address the problem or are some additional actions required?; and 3) if we had the right solutions the first time around, are these corrective/preventive actions still in place? There is, too, the possibility that the initial occurrences were misdiagnosed, which means that the actual ESD mechanism is still unknown and should be treated as something new.

A distinctly new ESD mechanism, on the other hand, would need answers to the following: 1) why haven't we encountered this before?; 2) how widespread is the problem? and of course, 3) what's causing it? The answer to question 3 must not contradict those of questions 1 and 2.

The answer to question 1 may (or may not) be one of the following: the device is new, there was a change in the device, there was a change in the manufacturing process of the device. The answer to number 2 can only be obtained by looking at real data, e.g., 1) by checking the test data of all 'suspect' devices for unexpected surges in certain failure modes; 2) by closely monitoring incoming FA requests from customers reporting the same failure mode; 3) by temporarily increasing the number of OQA samples to increase your confidence that 'good' devices are not being damaged by ESD prior to shipment; and 4) by performing other appropriate quality sampling schemes.

To answer question 3, one must be prepared to do a lot of activities, which include but are not limited to the following: 1) a thorough ESD audit of the line where the affected devices are being processed; 2) gathering of field meter readings from as many points on the production floor as possible to determine the highest static voltage present within it; 3) simulations to try to replicate exactly the same ESD failure from 'real-life' operations; and 4) a check of all materials contacting your units to ensure that none of them are non-ESD-protective. All of these activities must be geared towards finding where static charge build-up can occur and where such static charge build-ups can discharge uncontrollably.

Once the most likely root cause(s) has (have) been identified, corrective/preventive actions need to be implemented to eliminate it (them). Only when these actions are able to preclude the recurrence of the same ESD issue can one say that it has been resolved. It would be a good idea to undertake the activities mentioned above in a systematic manner, such as by employing TQM-based problem-solving methods or doing it under an 8-D process. After all, root cause analysis of ESD failures is one of the toughest tasks around, and it pays to be very systematic when dealing with it.