Читать книгу Design for Excellence in Electronics Manufacturing - Cheryl Tulkoff - Страница 46

MIL‐HDBK‐217, “Military Handbook for Reliability Prediction of Electronic Equipment,” paved the way for MTBF calculations. The handbook was based on statistical and actuarial research work done by the Reliability Analysis Center at the Rome Laboratory at Griffiths AFB, Rome, NY and became required due to Department of Defense backing. Widely used as a purchase requirement for all military electronic applications, it spawned several civilian versions. It contains “statistical” constant (random) failure rate models for electronic parts like ICs, transistors, diodes, resistors, capacitors, relays, switches, connectors, etc. The failure rate models were based on statistical projections of the “best available” historical field failure rate data for each component type. Many assumptions were used, including the belief that past data can be used to predict future performance. It is also known as the parts counting technique. To perform an MTBF calculation, select the failure rate for each part in the design from the historical data and apply a thermal stress factor intended to account for diffusion, a solid‐state failure mechanism that plagued early electronics (1960s–1980s). Then, calculate the R(t):

(2.1)

MTBF was intended to enable reliability comparison of different design alternatives, foster the use of the most reliable components, drive reliability design improvement and determine expected failure rates for military logistics planning (a key driver). However, MIL‐HDBK‐217 was eventually proven to be both inaccurate and misleading. It frequently caused design resources to be expended on non‐value‐added efforts. Key disadvantages of the method included:

 Assumption that past performance predicts future results: technology advancements change assumptions.

 The diffusion failure mechanism was designed out.

 The handbook was not updated to account for new /other failure mechanisms.

 It became obsolete quickly: field data could not be collected, analyzed, and logged rapidly enough.

 Increasing design complexity led to increased costs and time required to perform the analysis.

 Average constant failure rate data did not correlate to actual failures.

 It did not address infant mortality, quality, or wearout issues, where up to 65% of failures occur (O'Connor 2012).

The handbook was declared unsuitable for new designs and phased out in 1994. A University of Maryland study documented its flaws and proposed it be replaced by a reliability physics approach. Avoid the use of MTBF or MTTF as reliability metrics. Manipulating these numbers is easy due to the adjustment of multiple quality factors present in the model. MTBF and MTTF are frequently misinterpreted and provide a better fit for logistics and procurement activities.