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Why HALT Won't Give You an MTBF (And Why You Shouldn't Care)

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Describes the concepts of Highly Accelerated Life Test (HALT) and Mean Time Between Failures (MTBF) and why each is not related to the other.

Published in: Engineering
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Why HALT Won't Give You an MTBF (And Why You Shouldn't Care)

  1. 1. Why HALT Won’t Give You An MTBF And Why You Shouldn’t Care Mark L. Morelli Reliability & Test Engineer (Retired) Hobbs Engineering Webinar June 4, 2014
  2. 2. Author’s Bio • Recently retired after 32-year career in reliability and test engineering – Aerospace, military, and commercial building systems – Applied HALT > 200 products (> 500 separate testing activities) – Applied HASS to ~ 10 product lines (many thousands of units tested) • BSEE Univ. of Hartford • Adjunct Professor, Univ. of Hartford • Authored and presented numerous technical papers • Presently a freelance writer for The Motley Fool (finance site) with a focus on technology and innovation 2
  3. 3. Agenda • Discussion of MTBF • Why HALT doesn’t provide a MTBF value • Why HALT will improve reliability 3
  4. 4. Mean Time Between Failures (MTBF) • MTBF = elapsed time between inherent failures – Assumes a renewal process (system repaired upon failure) • Does not provide a failure distribution or pattern • Most prediction methods (e.g. MIL-HDBK-217) use “old” data MTBF = Cumulative Fleet Op Time ÷ Failures MTBF = time based parameter but does not account for product failure distribution/patterns 4
  5. 5. MTBF and failure distributions T (hrs) System 1 failures System 2 failures System 3 failures 50 x 100 X (2) 200 x 500 1000 2000 x 5000 10000 x 15000 x X (2) 20000 x X (2) All systems have same MTBF = 20,000 ÷ 4 = 5,000 hours 5
  6. 6. MTBF and failure distribution 6
  7. 7. Why HALT ≠ MTBF • HALT is exploratory process used on electronics that seeks to identify weaknesses though application of increasing and varying stress – Difficult to correlate to a precise time period but process does identify most failure types that occur during product life cycle – Difficult to calculate an acceleration factor between test and deployment • Typically a small number of test articles are used and not every sample will have all stresses (temperature, vibration, electrical, etc.) applied – Nearly impossible to accurately measure reliability w/ low sample sizes – My experience indicates that many (if not most) product failures are due to lot-related part defects or process variations that can not be found in a single test at a single point in time HALT = stress-based tool that addresses typical product failure types 7
  8. 8. Example: Capacitor failures 8 0 2 4 6 8 10 12 14 16 Nov-93 Mar-94 Jun-94 Sep-94 Jan-95 Apr-95 Jul-95 Oct-95 Feb-96 May-96 Aug-96 Dec-96 # Failures When Produced When Failed Design testing completed Jan 1994: No capacitor failures
  9. 9. What HALT does do • Makes product more robust – Can withstand (sometimes) unknown factory and field environments • Allows development of ongoing reliability tests (ORT), including production screening regimen (e.g. HASS) – Not all failures are related to design and will creep into product over time 9
  10. 10. 10
  11. 11. Ongoing reliability testing • Needed to account for issues creeping into product over time – Lot-related (e.g. capacitor) problems – Process (e.g. solder) variation • Periodic re-HALT • Production screening – HASS 11
  12. 12. Summary • MTBF is a time-based parameter that is unrelated to a failure distribution • HALT is a stress-based tool that is related to failure patterns (and distributions) 12
  13. 13. Contact information Mark L. Morelli mathman6577@gmail.com Twitter: @mathman6577 LinkedIn: Mark Morelli (Greater NYC area) 13

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