- questions most frequently asked by visitors of www.EESemi.com
How should intermittent failures be analyzed?
failures are failures that do not manifest themselves all the time.
The fact that they are sometimes there and sometimes aren't can make
them very difficult to analyze.
Just like any other FA,
the first thing to do when analyzing an intermittent failure is to
establish what the failure mode is. Once
the failure mode has been established, the minimum set of conditions
needed to make the failure mode observable over a reasonable amount of
time must be determined. The more
defined this set of conditions is, the easier the FA will be. The
objective of this exercise is to be able to make the failure visible for
a period of time that makes FA possible, since FA can not be performed
on a 'good' unit.
An understanding of these conditions also gives the analyst an insight
as to what mechanism or physical phenomenon is responsible for the
of caution though - in trying to make the failure mode observable for
FA, one must take care not to 'aggravate' the failure mode or change it
in any way. This means that the intermittency of the failure mode
must be preserved - the failure should neither become permanent nor
should the unit become permanently good.
A very common
cause of intermittent failure is a marginally detached first or second
bond that sometimes loses contact with its bonding surface.
Another common cause is a broken ball bond neck or broken wedge bond
heel, which can result in intermittent loss of contact between the bond
and the wire. Bond-related intermittent failures such as these are
relatively easy to analyze, since a simple curve tracer can verify the
failure mode, i.e., the affected pin is open if the failure is there and
normal if the failure isn't there. Furthermore, it is quite easy to make
intermittent bond failures manifest themselves using a heat gun or, in
some cases, a freeze spray can.
temperature of the package physically shifts its features relative to
each other, causing the intermittent bonds to open. These bonds
return to their previous state once the package has returned to room
temperature. Once an intermittent failure is verified to be most likely
bond-related (i.e., a pin alternates between being open and normal with
the application of a high or low temperature), post-decapsulation internal inspection of the units
can be performed to
reveal the affected pins to have one of these bond-related defects.
failures that require electrical bias to manifest are more difficult to
analyze than bond-related issues. This is because a simple curve tracer
is not enough to make the failure observable - the unit must first be
set up in its normal bias condition. Furthermore, such failures are
die-related most of the time, so FA requires more than just visual
inspection of the device and package features.
Again, it is necessary to
know the minimum set of conditions needed to make the failure mode
observable for a reasonable amount of time. Many
non-bonding-related intermittent failures also manifest with the
application of extreme temperature (usually elevated) to the device.
Other failures may become visible with the application of bias
conditions bordering on datasheet spec limits, application of bias in
the presence of a high temperature, or exposure of the die to light or
analyst has determined these conditions and has set them up, the device
should exhibit its failure and make the conduct of failure analysis on
it possible. Failure analysis from thereon will simply be the same
as conventional failure analysis for a permanent failure.
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