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October 16, 2008

Electronics Reliability Prediction

These were some of the questions i had on reliability and got answers from various sources on web.

What is reliability?

The discipline of reliability engineering functions to analyze the expectedor actual reliability of products, processes or services, and identifiesactions that will reduce failures and/or mitigate their effects.

What is reliability prediction?

For our purposes today, the reliability prediction is simply the analysisof parts and components in an effort to predict the rate at whicha system or subsystem will fail.

Why do I need to perform a reliability prediction?

Reliability predictions are necessary to determine whether a product has an acceptable MTBF Additionally, the predictions will serve to identify weaknessesthat can easily be strengthened prior to the product introduction.

What are the different models used for reliability analysis?

Models used for electronic components are MIL-HDBK-217 or Bellcore

What is MIL-HDBK-217?

MIL-HDBK-217 was the original standard for electronic reliability predictions. It was designed by the military after WWII to provide reliability mathmodels for electronic devices. It is used by both commercial companies and the defense industry.

What is Bellcore?

The Bellcore reliability prediction model was developed by AT&T Bell Labsby modifying the equations from MIL-HDBK-217 to better represent whattheir equipment was experiencing in the field. The models are very similar,but Bellcore added the ability to take into account burn-in, field, and laboratory testing.The most recent revision of the Bellcore Reliability Prediction Procedure is TR-332.

Which model should I choose?
It depends! Based on the requirements of the reliability prediction analysis,it may be found that one model is more applicable under certain circumstances than the other. There are numerous differences between the two models.However, when comparing the two models, it is often helpful to simply review the fundamental differences.

What are the Differences between Bellcore and MIL-HDBK-217?

Difference #1: Recognition and Acceptance
Bellcore is primarily accepted in the United States. Because MIL-HDBK-217 was the original standard for reliabilityprediction analyses, it is known and accepted world-wide.

Difference #2: Concentration

The Bellcore model was designed with a focus on telecommunications equipment.On the other hand, MIL-HDBK-217 is geared towardsboth military and commercial equipment, and has no specific market focus.

Difference #3: Calculations and Equations
The basis of the Bellcore and MIL-HDBK-217 calculations are very similar.However, it is often found that the Bellcore calculations are optimistic moreoften than MIL-HDBK-217. Additionally, the Bellcore calculations generally require fewer part parameters for components reliability calculations.Conversely, MIL-HDBK-217 calculations are sometimes more pessimistic,and generally more part parameters are required for accurate calculation.This doesn’t necessarily suggest, that Bellcore failure rates will simplyalways be better. It just points out that, depending on the component types being analyzed, significant differences in failure rates may be predicted.

Difference #4: Consideration of Test Data
The Bellcore model now includes the additional capability of consideringburn-in data, laboratory test data, and field data. This feature is extremelyuseful in calculating failure rates that are based on historical data,rather than only considering stress data. Additionally, burn-in data is used to quantify the 'first year multiplier', which is an indication of infant mortality.

Difference #5: Multiplier
The Bellcore model expressed FITs (failures in time) in billion of hours.MIL-HDBK-217 uses millions of hours.

Difference #6: Parts
Bellcore provides models for gyroscopes, batteries, heaters, coolers,and computer systems that MIL-HDBK-217 doesn’t support. MIL-HDBK-217 provides models for printed circuit boards, lasers,SAWS, magnetic bubble memories, and tubes that Bellcore doesn’t.

Difference #7: Environments
Due to Bellcore telecommunications roots,the operating environments that Bellcore supports are limited. In the most recent issue of Bellcore, additional operating environmentsof 'Airborne, Commercial' and 'Space, Commercial' were made available. MIL-HDBK-217 has always offered a wide range of operating environments.Currently, MIL-HDBK-217 supports a variety of ground, sea, air, and space environments.

Difference #8: Quality Levels
Bellcore makes the assignment of quality levels much simpler.The current issue of Bellcore supports four standard quality levels (0 through 3).These quality levels are identical for all component types, and are based onsome generalities regarding the origin and screening levels. In MIL-HDBK-217, the quality levels that are used differ from one part type to another.The quality levels are traceable to various military standards for each part type and can be quite confusing.

Operating Environments for MIL-HDBK-217

  • Ground ,
  • BenignGround
  • FixedGround
  • MobileNaval
  • ShelteredNaval
  • UnshelteredAirborne
  • Inhabited
  • CargoAirborne
  • Inhabited
  • Launch
  • CargoAirborne
  • Uninhabited
  • FighterAirborne
  • Rotary WingedSpace
  • FlightMissile
  • LaunchCannon

Operating Environments for Bellcore

  • Ground
  • Benign Ground
  • FixedGround
  • MobileAirborne
  • Inhabited
  • CargoSpace
  • Flight

Bellcore-MIL HDBK 217 Comparison Conclusion

The Bellcore methods are simpler to use and therefore might be the first choice.However, in appliances we often use medium powered relays that aren’t very popular in the telecommunications equipment. As a matter of fact, Bellcore doesn’t have a medium powered relay model and MIL-HDBK-217 does.Therefore, it is recommended to adjust as necessary and remain consistent in the methods used.

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