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A Functional Approach to FMEA

Functional FMEA - When performing a design FMEA there are various ways in which
you can approach the system that you are analyzing. The system tree or project tree
that you set up for performing the FMEA can mimic your hardware, functions,
interfaces or a combination of these things.
This functional FMEA approach can still provide us with the information that we need,
but the analysis can possibly be conducted in a fraction of the time involved with the
piece-part approach. This same approach might be used for interface functions,
memory, input/output or other types of functions.

The top level board functions that we are modeling in our FMEA would have failure
rates assigned to them that correspond to the failure rates of the parts or portion of
parts that contribute to the function. Some of the better FMEA tools on the market even
have a means to easily link functions in the FMEA to parts or portions of parts in their
reliability prediction. This way as the reliability prediction is updated the analyst can
quickly refresh the failure rates associated with the functions defined in the FMEA.
Obviously, if any of the functions have failure modes resulting in critical effects, then
the analyst would most likely take a closer look at the parts involved in the critical

Hopefully, this information on functional FMEA has been helpful, and has started you
thinking of other ways that this powerful analysis technique can be used in assessing
your designs.
For example, let's take a look at how we might
approach the FMEA for a circuit card assembly
(CCA). One approach might be to perform a
piece-part level FMEA where we look at  ways in
which the CCA might fail and its effect on the rest
of our system. Then, we could look at how each
part is most likely to fail and the resulting effects of
each of these part failure modes on the CCA. A
good resource when identifying piece-part failures
is the
Reliability Analysis Center's FMD-97 Failure
Mode Distributions guide. Obviously, if our CCA
has very many parts this hardware approach could
become very tedious and time consuming. For
instance, we could have many capacitors on our
board, each with several failure modes, which
possibly result in the same effect on our CCA.
Although this approach is quite thorough, much of
this repetitive effort may not give us a great return
for the time spent.
hardware piece-part FMEA
A common approach used in this situation is to model the CCA functionally, so that the
top level functions that the circuit card performs are listed instead of the components.
Each function represents a group of parts or portions of parts, which contribute to the
function. We then list the failure modes that are applicable to the function and the
effects these failure modes have on the CCA.
functional FMEA
For example, let's say that we have  20
decoupling capacitors involved in our
design, whose failures result in the same
effect on the CCA. In this case, we could
define a single function in our analysis tree
called Decoupling Capacitors. We would
then define the failure modes associated
with this function and their effects on the
CCA. In this simple case, we have one
function with three failure modes to analyze
instead of 20 capacitors, each with three
failure modes.
fmea fmeca

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