FMEA, FMECA Click here for MTBF

Failure Analysis & Troubleshooting

A FMECA ("Failure Mode Effects Criticality Analysis") consists of an outlining of all of the possible Failure Modes of all elements in a system, and a determination of the Effects and Criticality of these failure modes. This analysis looks at the failure of each element within a device or system to determine the effect on the end performance. The failure of an element may be simple or complex. As an example, a resistor has three failure modes. The resistor could be open, shorted, or outside of its specified tolerance. An integrated circuit is obviously much more complex.

The goal is to eliminate any single-point failures that would cause the system to fail to meet its performance requirements. Another way to look at this is to validate that a “graceful” degradation of a device or system will take place over time.

As a further example, suppose that the input to a device or system is coupled to the outside world, via a resistor. If the resistor were to open, there would be no output from the device or system. The effect is, therefore, non-performance, the criticality is very high, and this constitutes a single-point failure. The goal is to minimize the likelihood of single-point failures and to provide the greatest amount of graceful degradation.

This analysis is performed using specialized computer software, along with SPICE and other mathematical models of the system.

The FMECA begins with a stress-based reliability (MTBF) analysis. This generates the failure rate of each of the components. Each component in the BOM is assigned a set of failure modes. These are normally defined using MIL-HDBK-338B which has a definition of the common types of failures for each type of component along with the probability (Mode Failure percentage) of each failure mode occurring. For example: Open 30%, Short 30%, out of tolerance 40%.

Next, each failure mode is analyzed for its local, next higher assembly, and system level effect which will occur when the failure model happens as shown below.

The failure mode is also assigned a severity classification.

Severity Type Description

The Mode criticality is then computed. Mode criticality is a numerical value that can be assigned to each failure mode. Mode criticalities are based on the FMECA approach defined in MIL-STD-1629 and other guides. The Mode Criticality is then computed as:

Mode Criticality = Failure Effect Probability * Mode Failure Rate * Operating Time of the System

This is really the essence of the FMECA. The result is the ranking of the most critical faults by the probability that they are most likely to occur -- the worst failure mode offenders. The results are listed in a table or graphed as shown below.