Methodology

The Equations
Failure Rate, MTBF, and FITs
 

Description of Methodology

The parts count method is a technique for developing an estimate or prediction of the average life, the Mean Time Between Failures (MTBF), of an assembly.  It is a prediction process whereby a numerical estimate is made of the ability, with respect to failure, of a design to perform its intended function.  Once the failure rate is determined, MTBF is easily calculated as the inverse of the failure rate, as follows:

The general procedure for determining a board level (or system level) failure rate is to sum individual failure rates for each component.  For MIL-HDBK-217, the summation is then added to a failure rate for the circuit board, which includes the affect of solder joints. Component failure rates are provided by MIL-HDBK-217, "Military Handbook, Reliability Prediction of Electronic Equipment", as standard part failure rate models or directly from the manufacturers.

The failure rates presented apply to equipment under normal operating conditions, i.e., with power on and performing its intended function in its intended environment.  Consideration is given to various environments, component quality, and thermal aspects. 

The Equations
A sample calculation for integrated circuits taken from MIL-HDBK-217 is as follows:

Each factor in this equation is dependent upon a certain part parameter. The end result of this equation is the failure rate of the integrated circuit.
   

Failure Rate, MTBF, and FITs

For this discussion, we will assume that the resulting failure rate is shown in failures per million hours. This is simply the number of failures that you would expect to have in a million hours of operation of your equipment. Failure rates for many basic devices are well below 1 failure per million hours, so these values may seem insignificant. But if you have hundreds of parts in your design and have a thousand systems operating in the field, you can see that the failure rates will quickly add up. MTBF, or Mean Time Between Failures, is the inverse of the failure rate and is the average time between failures. It is calculated from the failure rate as follows:

You can choose the units in which the failure rate is shown. Another common unit used, besides failures/million hours, is failures per billion hours which is also known as FITs (Failures In Time).

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