Ask the RAC

By: William K. Denson

RAC responds to numerous technical inquiries on reliability and maintainability topics. Questions and answers of general interest to the R&M community are published in each issue of this journal. The RAC's technical inquiry hotline number is (315) 337-9933.

Question:

When using a failure mode distribution from sources like RAC's "Failure Mode/Mechanism Distribution" document (FMD-91) in a FMECA and the failure mode "drift" or "change in value" is given, what is the exact percentage change that these modes represent? For example, if a composition resistor distribution indicates that 20% of failures are due to drift, what percentage resistance drift do 20% of the devices exhibit?

Answer:

Since the RAC collects the majority of its data from external data sources and published information, the failure modes and mechanisms listed in FMD-91 are empirical modes reported by those from which RAC has collected the data. For example, if the maintenance technician performing the troubleshooting function had perceived the cause of a circuit failure to be a resistor whose value was such that it resulted in inadequate circuit operation, the failure mode listed may be "drift." The exact percent drift is unknown, other than the fact that it changed sufficiently to cause circuit failure in its specific application.

Likewise, the short and open categories, are cases in which:

the drift was sufficient to cause the maintenance technician to perceive a catastrophic open or short or
a dead short (zero ohm) or complete open (infinite impedance) was realized.

Another source of data used to derive failure mode distributions is life test results. For these sources, component performance is usually monitored periodically and the mode of failure is more definitive than data collected from field usage. In these cases, more precise definitions of failure are usually known, although for the purposes of FMD-91, all "drift" failures would be combined, regardless of percent change.

To better quantify drift characteristics of components, the RAC is establishing a database devoted solely to the electrical parameter change as a function of time/temperature and other applicable stress unique to specific component types. Math models will then be derived which will allow users of this data to predict parametric change under specific operating conditions. This database and the resulting models are to be presented in RAC's "Reliable Application of Components" series.

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From the Reliability Analysis Center Webserver. Comments or questions can be submitted using e-mail or telephone at (315) 337-9933. (Last update: December 1, 1993, jmc)