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

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. 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
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3. Agenda
• Discussion of MTBF
• Why HALT doesn’t provide a MTBF value
• Why HALT will improve reliability
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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
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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
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6. MTBF and failure distribution
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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. 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. 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
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10. 10

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
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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)
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13. Contact information
Mark L. Morelli
mathman6577@gmail.com
Twitter: @mathman6577
LinkedIn: Mark Morelli (Greater NYC
area)
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