# What are FIT's and how they used in reliability calculations?

In Military, Medical, Space, Professional eqt. design there is a need to be able to prove that your device can last a certain length of time with a certain confidence level. Or that reliability must be used in design to inform the design direction, either through component selection, component testing and sort or in amelioration techniques (like redundancy, FEC's - Forward Error Correction etc.).

How are FIT's (Failure In Time) used in the reliability aspect of design and verification? Examples of calculations?

How are FIT's determined/derived?

How is this related to MTTF (Mean Time To Failure) and MTBF (Mean Time Between Failures)

• You can never prove a design will last a certain time. It's all a probability game. You can calculate with some confidence how long something is likely to last on average, but not that any particular unit will last some minimum time. – Olin Lathrop Mar 4 '13 at 20:01
• @OlinLathrop edited to better reflect probabilistic aspects. – placeholder Mar 4 '13 at 20:10
• Look at IEC 61508. – starblue Mar 4 '13 at 20:17

## 3 Answers

The term FIT (failure in time) is defined as a failure rate of 1 per billion hours. A component having a failure rate of 1 FIT is equivalent to having an MTBF of 1 billion hours. Most components have failure rates measured in 100's and 1000's of FITs. For components, such as transistors and ICs, the manufacturer will test a large lot over a period of time to determine the failure rate. If 1000 components are tested for 1000 hours, then that is considered to be equivalent to 1,000,000 hours of test time. There are standard formulas that convert the number of failures in a given test time to MTBF for a selected confidence level. For a system of components, one method of predicting the MTBF is to add the failure rates of each component and then taking the reciprocal. For example, if one component has a failure rate of 100 FITs, another 200 FITs and another 300 FITs, then the total failure rate is 600 FITs and the MTBF is 1.67 million hours. For military systems, the failure rates of each component can be found in MIL-HDBK-217. This document includes formulas to account for environmental and usage conditions such as temperature, shock, fixed or mobile equipment, etc. In initial stages of a design, these calculations are useful in determining the overall reliability of a design(to compare with the specified requirement) and which components are most significant in terms of the system reliability so that design changes can be made if deemed necessary. However, component reliability is more of an art than a science. Many components are so reliable that it is difficult to accumulate enough test time to get a good handle on their MTBF. Also, relating data taken at one set of conditions (temperature, humidity, voltage, current, etc.) to another is open to large errors. As already mentioned in the comments, all of these calculations are mean numbers and are useful in predicting the reliability of a large number of components and systems, but not any individual unit.

• +1 for the answer. But I will note "However, component reliability is more of an art than a science" is not true. This is driven by hard science in the form of the Arrhenius equation and Activation energy of failures modes. the fact that it is statistical doesn't mean there isn't science behind it, in fact there is zero room for guessing as demonstrated by the Mil-handbooks. – placeholder Mar 5 '13 at 17:32
• I disagree strongly. Reliability figures for systems calculated from MIL handbooks are notoriously inaccurate. Any reliability numbers obtained from accelerated life testing are subject to large errors because components do not necessarily obey the acceleration laws. MIL-HDBK-217 is no longer used for new system reliability calculations. – Barry Mar 6 '13 at 0:41
• I agree with Barry. The problem with Activation Energy and similar formulas is that the experimental data to fit formulas are usually missing or vague and vanilla formula are used without evidence that the parameters are valid in the specific case. Moving from 1000 Hours test at high stress and calculating the working lifetime in 15 years is sometime more faith than experimental evidence. – matzeri Jul 14 '16 at 21:31

I understand FIT as Failures over a billion hours of operation.

MTBF = 1,000,000,000 x 1/FIT JEDEC JESD85 (Standart Used for semiconductors and thus relevant for most electronics)

We use for our (industrial electronics) reliability calculations Siemens SN 29500, but it is kinda specific for Europa.

• Welcome to EE.SE. When quoting standards such as FIT you need to back it up with links and/or quoted comments from official sources. – user105652 Jul 14 '16 at 20:54
• @Sparky256 SN 29500 is a quasi Standart. But anyway FIT is defined in JEDEC JESD85 (Standart Used for semiconductors and thus relevant for most electronics) – marco wassmer Jul 15 '16 at 7:36

There is some truth to both of your answers. The environment that the device will see is a factor along with the type of packaging technology (ceramic vs plastic packaging). These items were not part of normal MIL-STD-217.

When we were trying to use mil-std-217 for automotive electronics, we had a PHD statics person that would correlate lab accelerated testing with field experience. The he would recommend factors( I remember things like technology, new IC vs Old IC, environmental factors) that would be used in the calculation.

Not sure what is done in this area today as I have been out of the reliability field for some now.