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Falsely Induced by Rel Testing
Can a reliability test induce false failures?
What failure mechanisms can be falsely induced by reliability testing?
to assess the reliability of devices are designed to make sample units
fail by subjecting the parts to conditions that accelerate specific
failure mechanisms. What the reliability engineer must understand
though is that reliability testing, if not performed properly, can
induce false failures not representative of the true reliability of a
population of devices. Needless to say, a reliability engineer
must be aware of what these false failures are and how they can be
the ability to subject parts to high temperature in the presence of
electrical excitation, has long been used to accelerate oxide failures
and transistor degradations.
Burn-in, however, requires extensive handling of the samples, since
these need to be loaded onto and unloaded from burn-in boards.
Burn-in can therefore induce false failures related to loss of
electrical contact due to lead deformation. Burn-in can likewise
result in EOS issues, i.e., when the
burn-in board or a power supply used is questionable.
A few of the
industry-standard reliability tests such as HAST and THB subject parts to moisture,
and therefore require deionized (DI) water as utility input. Unfortunately,
the inadvertent use of contaminated water as input can result in metal
corrosion in the samples, whether at package- or die-level. Thus,
corrosion issues found after reliability testing that affect both the
qualification and control samples must prompt the reliability engineer
to check the DI water used, just to make sure that the corrosion
failures were not simply due to the input water used.
ESD is another failure mechanism that can be
falsely induced by the reliability testing itself. The preparation and
handling of samples, if not done in an ESD-preventive manner, can cause
the samples to fail by, well, ESD. Thus, ESD controls that are
implemented in production lines must also apply to the reliability lab.
intermetallic growths and
excessive oxidation of metal surfaces, on the other hand, can result from excessive
environmental chamber temperatures. An equipment or set-up problem
that results in a higher temperature inside the chamber than what's
required will falsely accelerate the mentioned failure mechanisms.
Discoloration of the samples are obvious signs of exposure to excessive
temperatures, but this may not always be present. Failures due to
excessive intermetallic formation must therefore trigger the reliability
engineer to check if the oven used is providing the correct thermal
stress to the samples.
just some of the common failure mechanisms falsely induced by the
reliability testing itself. Every reliability engineer must
therefore consider the possibility of failures that are falsely induced
by the rel test itself whenever conducting investigations on
qualification or reliability monitor failures.
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