- questions most frequently asked by visitors of www.EESemi.com
Root Cause of an ESD Problem
How can one
determine the root cause of an ESD issue?
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
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.
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.
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
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.
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.
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