Short description of the principles of Adaptive ESS

1. Adaptive Environmental
Stress Screening
Hilaire Ananda Perera

2. Environmental Stress Screening (ESS)
Environmental stress screening of a product is a
process which involves the application of one or
more specific t
ifi types of environmental stresses f
f i tl t for
the purpose of precipitating to hard failure,
latent, intermittent, or incipient defects or
flaws which would cause product failure in
the use environment. The stress may be applied
in combination or in sequence on an accelerated
q
basis but within product design capabilities.
Hilaire Ananda Perera
AP – 13 Sep 2006
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3. Should the ESS Chosen Remain the Same
The form of ESS chosen by the equipment
manufacturer is dependant on the failure
mechanisms for the relevant field failures
The ESS process is dynamic, and must change as
product failure behavior changes
For this reason, it is not appropriate to “Spec” an
ESS regimen and leave it unchanged throughout
product life
Hilaire Ananda Perera
AP – 13 Sep 2006
.
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4. What Required from the ESS Process
The ESS Process should detect defective devices
without consuming an unacceptable portion of the
other “
th “good” d i ’ useful lif
d” device’s f l life
The more flaws an equipment has the higher is its
has,
failure rate
Large Flaws Early Failures (Infant Mortality)
Small Flaws End of Life Failures (Durability)
ESS should remove Large Flaws without
effecting much of the Small Flaw population
Hilaire Ananda Perera
AP – 13 Sep 2006
.
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5. Adaptive Environmental Stress Screening
ESS is a process rather than a test in the normal
p
accept/reject sense. Adaptive ESS is based on the
adjustment of screens in response to previously
observed screening results to minimize
Outgoing Defects.
With no firm failure mechanism/mode information,
Random Vibration followed by Thermal Cycling with few
Power On/Off cycles is a good default condition.
y g
Screening should not stress the equipment such that
fatigue failures are precipitated
Hilaire Ananda Perera
AP – 13 Sep 2006
.
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6. ESS Strength Estimation with no Failure/Time Data
The Screening Strength of a given stress screen profile is defined as the
probability that the stress screen will precipitate a latent defect into a
detectable failure, given that a defect is present. Screening provide assurance
on the Outgoing Reliability
Screening Strength for Temperature Cycling (STn) is a function of Temperature Range =T;
Temp. Rate of Change =R; Number of Cycles =n
= 1 − e[−[0.0017⋅(T + 0.6) ⋅ln(e + R) ⋅n]]
0.6 3
(
STn
Screening Strength for Random Vibration (SVt) is a function of G = gRMS; Vibration Duration = t
= 1 − e[−(0.0046⋅G ⋅t )]
1.71
SVt When T = 111oC; R = 10oC/Min
n =16 Cyc; G = 5gRMS;
Combined Screen Strength (SS) = 1 - (1-STn).(1-SVt) t = 15 Min
SS = 0.999826
The Screening Strength equations were developed by Hughes Aircraft Company, and modified by Rome Air
Development Centre (RADC) based on the data from McDonnel Aircraft Co. and Grumman Aerospace
Corporation
Hilaire Ananda Perera
AP – 13 Sep 2006
.
6

7. Chance Defective Exponential (CDE) Model
CDE Model is based on the assumption that the
population of components within like equipments is
comprised of two subpopulations i.e. A main
subpopulation of “good” components and a much
smaller subpopulation of defectives
defectives.
The defectives contain major flaws which degrade
with stress and time and are manifested as early-life
early life
(infant mortality) failures. The failure rate of a
defective part is several orders of magnitude greater
than the failure rate of a “good” part
g p
More Information Available in MIL-HDBK-344A
Hilaire Ananda Perera
AP – 13 Sep 2006
.
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8. Equation Selection from SigmaPlot
Chance Defective
Exponential (CDE)
Model
Hilaire Ananda Perera
AP – 13 Sep 2006
.
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9. Outgoing Reliability Assurance
C = Average rate of defect precipitation under a given set of stress conditions
B/C = Incoming Defect Density (Din)
A / Normal Failure Rate = Acceleration Factor (AF)
Accelerated Life Tests
-
Environmental Stress Screening
Screening Strength (SS) = 1 – e -Ct
C
ESS Test Strength (TS) = SS * Detection Efficiency (DE)
Outgoing Defect Density (Dout) = Din * (1 - TS)
X
-Ct
Time To Remove 99.999% Defects = (-1/C)*ln(0.00001)
Rate (FR)
FR(t) = A + Be
X
X
re
(
Failur
Chance Defective Exponential Model
X
X
Time
t1 (variable)
Gather failure data and establish failure rate/time distribution functions
• Starting t1 is based on equipment parts count. t1 is varied depending on the effectiveness of the
Stress Screen ( Adaptive ESS ).
• Proper screening levels assure that there are no manufacturing process related failures and latent
defects (i.e. ESS + FFT). FFT = Failure Free Time
• Performance of FFT is an accurate precursor to the kind of reliability to be expected in the field
Hilaire Ananda Perera
AP – 13 Sep 2006
.
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10. Benefits of Adaptive ESS
• Able to produce High Confidence Reliability Numerics for
Reliability Guarantees
• Minimize Warranty Failures (move failures from field to factory)
Using Optimum Outgoing Defect Densities
• Able to apply Failure Rate/Time Distributions to Predict
Reliability of New Similar Electronic Controllers
• Possibility to Determine Cost Optimized ESS Times
• Improved Overall Quality of the Process and the Product
• Reduced Loading on Screening Facilities
g g
Hilaire Ananda Perera
AP – 13 Sep 2006
.
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