Multiple Environment Over Stress Testing
(MEOST)
A Primer with Examples from an Automotive
Component Supplier
October 26, 2011
John Kreucher
Delphi Powertrain
Auburn Hills, Michigan
810-577-9344
Multiple Environment Over Stress Testing
(MEOST)
Acknowledgements:S. Shaffer
B. Phegley
F. Couillot
A. Kleyner
AGENDA:
• Accelerated Testing Basics
• Time Compression
• Accelerated Life Testing (Constant Stress)
• MEOST
• HALT
Multiple Environment Over Stress Testing (MEOST)
A Primer with Examples from an Automotive Component Supplier
Accelerated Testing BasicsFor Field-Correlated Tests
• Analytical models must align with physics of usage/damage and environmental loading
• Failure modes under accelerated conditions must correlate to field failure modes
• No “unnatural” or foolish failure modes
• Shape parameter at each stress level (e.g. Beta for 2-P Weibull) should be the same
• Normally, to correlate an accelerated test, we “chase” a particular failure mode of
concern
• Other failure mechanisms occur at different rates and are disregarded analytically
• Reliability demonstration is possible, but assumes that part variation is representative
• Because of variability of field usage and environments, and uncertainty in material
constants, a precise acceleration model is impossible to obtain
• The best we can hope for is a useful test
1.Compress time
2.Increase the stresses that generate failures (accelerated stress testing)
� Elevated (Constant) Stress Testing
� Multiple Stress Level Testing (MEOST)
� Qualitative Methods
Problem: Available Test Time < Expected Product Lifetime
Solution::
TIME COMPRESSION
1. Remove non-damaging events
and/or
2. Accelerate the number of cycles per unit of time (apply stress
more frequently than it occurs in service)
TIME COMPRESSION
Automotive Trunk Lid Closure Test
10 cycles/hr x 100 hrs = 50 cycles/hr x 20 hrs = 1000 cycles
Five times as fast
for 1/5 of the time
Consider a test that opens and closes an automotive trunk lid 8000 times in 10 hrs…
What might you miss??
1.Compress time
2.Increase the stresses that generate failures (accelerated stress testing)
� Elevated (Constant) Stress Testing
� Multiple Stress Level Testing (MEOST)
� Qualitative Methods
Problem: Available Test Time < Expected Lifetime
Solution::
Stress – Strength Conceptual ModelR
ela
tive F
requency
Customer
Stress
Component
or Subsystem
Strength
Test Severity
(%tile User)
In testing, we usually work here
Q: Would you expect a higher field or test reliability?
A: Unless you are missing environments, or unless your parts do not
represent manufacturing variation, then:
Field Reliability > Test Reliability
Stress – Strength Conceptual ModelR
ela
tive F
requency
Customer
Stress
Component
or Subsystem
Strength
Test Severity
(%tile User)
In testing, we usually work here
• For the sake of conservatism, testing is often conducted at higher than nominal
levels, say 97.5 %-tile
• It’s OK to test at “only” 97..5 %-tile stress, because:
1. If your test has multiple environments, the likelihood of having extreme
exposures for each of the environments concurrently is extremely small, and
2. More mistakes are made by missing environments and/or misrepresentative
parts.
Accelerated Life Testing
Critical assumption: The same failure mechanisms will be
present at the higher stress levels and will act in the same
manner as at normal stress levels.
tn = time to failure under normal stress
ta = time to failure at high stress level
then tn = AF x ta where AF is an acceleration factor
Time to Failure
Str
ess
1. Life Distributions determined
experimentally at high stress levels
2. Stress model used to predict life
at normal use levels
Customer Usage
Level
“One Life”
Accelerated Life Testing
Accelerated Life Testing
Stress Models
� Inverse Power Law
� General mechanical stress model
� Thermal and mechanical fatigue (Coffin Manson)
� Arrhenius Model
� Chemical/material reactions driven by temperature
� Eyring Models
� Temperature + Other (Mechanical, Humidity, Current Density)
� Kemeny Model
� Temperature + Voltage
Case Study 2:
Accelerated Heater Cycling TestDELPHI PLANAR OXYGEN SENSOR (OSP)
• Stoichiometric Switching Sensor
• Heater Integrated within the Sensing Element
• In Production Since 2002
Sensing Element
Accelerated Life Testing
Example
• Test Plan: Test (40) pcs to failure, (10) ea. @ 21V, 22V, 23V, and 24V
• Failure Definition: Failure is resistance increase >/= 10% of original value, or open heater. Resistance checks once/day.
• Requirement: Elements must survive 50,000 thermal cycles at 13.5V
Case Study 2:
Accelerated Heater Cycling Test
DELPHI PLANAR OXYGEN SENSOR (OSP)
Accelerated Life Testing
Example
Case Study 2:
Accelerated Heater Cycling Test
DELPHI PLANAR OXYGEN SENSOR (OSP)
Accelerated Life Testing
Example
1.Compress time
2.Increase the stresses that generate failures (accelerated stress testing)
� Elevated (Constant) Stress Testing
� Multiple Stress Level Testing (MEOST)
� Qualitative Methods
Problem: Available Test Time < Expected Lifetime
Solution::
• Multiple environment test to failure after achieving one life
• All parts should fail on or before the last step of overstress (at the Destruct Limit)
• Combined environments allow for interactions which are potentially more
relevant than single environment tests
MEOST
Multiple Environment Over-Stress Test
MEOST
Multiple Environment Over-Stress Test
• Primarily Qualitative, but can be made “semi-quantitative” with some rigor
• Good alternative for newer product lines or significant product changes, when
failure mode(s) and failure times are uncertain
• Can be done on small sample size, therefore a good option when parts are
expensive
• Best done w/ special equipment (fast ramp chambers and pneumatic
shock/vibe equipment), but can be very usefully administered on standard
equipment
MEOST
Test Time
Co
mb
ine
d T
es
t S
tre
ss
100%
One Life
Destruct Limit
MEOST PROCESS
1. Define Operating Rectangle for each Environment
2. Establish Destruct Limit for each Environment (measure or estimate)
3. Calculate Overstress Levels (stress and time steps)
4. Conduct Test to Failure (fix design if failure occurs within one life)
5. Plot Weibull results as % Stress-Time
1
2
3
MEOST
Operating Rectangle
1. Which environments are present? Which might interact to cause relevant failure modes? Conversely, too many environments lead to test complexity and cost.
2. Instrument your product in the field and record data for each important environment.
3. Analyze data to determine the (say) 97.5th %-tile user : The “Design Level”
4. Define the operating profile and one life on test.
Easy Level
MEOST
Potential Environments:
• Vibration (random, sine)
• High Temperature
• Low Temperature
• Temp Cycling
• Humidity
• Pressure
• Voltage
• Power Cycling
• Mechanical Actuations
• Etc.
MEOST
Conduct Test to One Life
1. Test 10 to 12 randomly selected units for one life within the operating rectangle.
• You should expect to pass!
2. Find and fix any failures, and restart the test.
• This might include design or quality improvements
3. Proceed to step stress testing with the same parts.
MEOST
Test Time
Co
mb
ine
d T
es
t S
tre
ss
100%
One Life
Destruct Limit
MEOST PROCESS
1. Define Operating Rectangle for each Environment
2. Establish Destruct Limit for each Environment (measure or estimate)
3. Calculate Overstress Levels (stress and time steps)
4. Conduct Test to Failure (fix design if failure occurs within one life)
5. Plot Weibull results as % Stress-Time
1
2
3
MEOSTDestruct Limit
• The Destruct Limit (or Foolish Failure Limit) is the stress level where a product is
expected to fail with a high degree of certainty.
• The associated failure mode can be either related or unrelated to the failure
mode expected in the field environment.
• Example 1: Plastic case melting at 180 deg C.
• Example 2: PCB cracking at 10 Grms random vibration.
• Destruct Limits are best determined by testing parts under each environment
separately. Otherwise, analysis may be used (e.g. solder melt temp).
• Sometimes it is difficult or impossible to determine the Destruct Limit for a particular
environment due to test equipment limitations
• This may or may not compromise the integrity of the test plan
MEOST
Test Time
Co
mb
ine
d T
es
t S
tre
ss
100%
One Life
Destruct Limit
MEOST PROCESS
1. Define Operating Rectangle for each Environment
2. Establish Destruct Limit for each Environment (measure or estimate)
3. Calculate Over-Stress Levels (stress and time steps)
4. Conduct Test to Failure (fix design if failure occurs within one life)
5. Plot Weibull results as % Stress-Time
1
2
3
Over-Stress StepsCommon Mistake:
0 3.2 15 23 115 150
Vibration (grms) High Temperature (C)
Design
LevelDesign
Level
Destruct
LevelDestruct
Level
Easy
Level
Easy
Level
• It is not appropriate to overstress each environment to its Destruct
Level.
• Doing so may result in failure mode(s) not representative of your
product and application.
• In this example, the product is much closer to failing for
temperature than for vibration under normal usage conditions.
l l l l l l l l l ll l l l l l l l l l
Over-Stress StepsCalculate Over-Stress Step Size for each environment:
1002
1 ∗∆
∆=% OVER-STRESS =
Destruct Limit - Design Limit
Design Limit – Easy Limit=
-
Operating
Range
∆1
∆2
Str
ess
Destruct Limit
time
Design Limit
Easy Limit
100∗
The Lowest % Over-Stress will be used to compute the step size for each stress
and to determine the time interval for each Over-Stress Step.
Over-Stress StepsExample: Electronic Component Test
Standard Validation Test:
• Temp Cycles between -40C and 115C
• Random vibration profile with energy of 3.2 grms
• One Life on this test is considered to be 400 Hrs
1. Operating Rectangle
• Stresses (Easy - Design Limit):
• High Temperature (23C - 115C)
• Vibrational Energy (0g - 3.2g)
• Low Temp: Held at -40C
• One Life = 400 Hrs
2. Destruct Limits:
• High Temp: 150C (softening temperature of plastic case)
• Vibration: 15grms (based on vibration test in worst-case orientation)
Over-Stress Steps – Electronic
Component Example
3. Over-Stress Levels:
High Temperature:
38 %
LOWEST OVERSTRESS
% OVERSTRESS =150 - 115
115 - 23
-=
Vibration:
369 %% OVERSTRESS =15 - 3.2
3.2 - 0
-=
For each environment, the step size will be 3.8% of the Operating Range (38% ÷ 10 Steps)
Over-Stress Steps – Electronic
Component Example
Over-Stress Step Size:
(Stress and Time)StepsofNumberDesired
StressOverLowestRangeOperating
% −×
• Temp: (115 – 23) x 38%10
= 3.5C
• Vibe: (3.2 – 0) x 38%10
= 0.12g
• Time: 400 Hrs x 38%10
= 15.2 Hrs
Total Test Time =400 + 152 = 552 Hrs
Operating Rectangle:
400 hrs
Temp Cycles: -40 to 115 °C
Vibe: 3.2 GRMS
STEP 1
STEP 2
STEP 3
STEP 4
STEP 5
STEP 6
STEP 7
STEP 8
STEP 9
STEP 10High Stress Limit: 150°C (Destruct Limit), 4.4gRMS
3.5°C
0.12 gRMS
15.2 hrs
MEOST Plan:
Over-Stress Steps – Electronic
Component Example
MEOST
Test Time
Co
mb
ine
d T
es
t S
tre
ss
100%
One Life
Destruct Limit
MEOST PROCESS
1. Define Operating Rectangle for each Environment
2. Establish Destruct Limit for each Environment (measure or estimate)
3. Calculate Over-Stress Levels (stress and time steps)
4. Conduct Test to Failure (fix design if failure occurs within one life)
5. Plot Weibull results as % Stress-Time
1
2
3
Multiple Environment Over Stress Test (MEOST)
Q: So how do you make a Weibull plot of the failures if each overstress step involves an
increase in stress as well as time?
A: If stress and time are normalized to “100%” of the O.R., and if overstress steps are equal
increments of %stress and %time, then failure data can be plotted on a Weibull Stress*Time
plot…
Step Stress Test
Test Time
Co
mb
ine
d T
est
Str
ess
One Life
Stress Limit
x
x
xx
x
100% time
10
0%
Str
ess
0.01
0.05
0.10
0.50
1.00
5.00
10.00
50.00
99.00
1.0
STRESS TIME PERCENT
PE
RC
EN
T F
AIL
ED
β=31.94, η=1.52, ρ=0.92
1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0
2.0
1 Life
MEOST – Example
Delphi Probe Air Meter
Standard “Hot Vibe” Test:
• 3 Grms with Underhood Chasis PSD
• -40 to 125C Thermal Cycles
• 6 hrs/axis
Problem:
• Parts rarely fail this test
• Would like to learn the failure mode(s)
on test and the margin to customer
requirement.
Solution:
• Conduct MEOST
MEOST – Example
Air MeterStresses:
� High Temperature: The 97.5%-tile field temperature at the hybrid
circuit board was measured to be 65ºC. Destruct limit = 183ºC for
eutectic Sn/Pb solder.
� Vibration: The “Underhood Chassis” vibration spectrum is appropriate
for this Air Meter. One life is represented by 6 hrs/axis with this profile at
3 Grms. Destruct limit was determined to be more than 38 grms on test.
Equipment limitation prohibited exact determination.
� Note 1: Low temperature has not been observed to cause field concerns, as seen on
warranty and other customer returns. It will be fixed at -40ºC.
� Note 2: Humidity will not be comprehended in this test because humidity related failure
modes have not been observed in field returns.
MEOST – Example
Air Meter
Over-Stress Levels:
%274 %1002265
65183 %100 =
−
−=
−
−xx
EasiestDesign
DesignDestructHigh Temperature (ºC):
%1671 %100 03
338 %100 =
−
−=
−
−xx
EasiestDesign
DesignDestructVibration Level (grms):
Lowest Stress Increase
Step Size:
MEOST – Example
Air Meter
Test Plan:
1. Test 9 parts total.
2. Run operating rectangle. Rotate parts every 6 hrs: 6 hrs/axis = 18 hrs total.
3. Increase part stresses simultaneously according to step size determined above. Run each step for 1.6 hrs/axis.
4. Continue to increase part stresses, until failure or until all units reach destruct limit (10 steps).
Test Time
Co
mb
ined
Str
ess
1
25
No. of Failures
100%
18 Hrs
Destruct/Test Limit
(183C; 11.2 g's)
MEOST – Example
Air Meter
Step Stress TimeStress Time
Percent
First Anomaly
S/N
O.R. 1.000 100.0
1 1.274 127.4
2 1.548 154.8
3 1.822 182.2
4 2.096 209.6
5 2.370 237.0
6 2.644 264.4
7 2.918 291.8
8 3.192 319.2 901
9 3.466 346.6 898, 914, 480, 476, 926
10 3.740 374.0 906, 478
100.00 1000.00
1.00
5.00
10.00
50.00
99.00
µ
σ
σ
Stress*Time
Un
relia
bili
ty,
F(t
)
One Life
Fo
oli
sh
Fail
ure
Lim
it
MEOST – Example
Air Meter
Lognormal Distribution of
Stress*Time Intervals
(Soft Failures)
MEOST – Example
Delphi Multec Fuel InjectorF. Couillot
Standard Pressure Cycling Test:
• Fluid Pressure Cycling 3 to 250 bar
• Pressure Wave Frequency - fixed
• Temperature: Ambient
• Test Fluid: Oil
• Target Life: 10M Cycles
Problem:
• Prior design did not meet customer requirements
• Lengthy Test and Fix loops
• No efficient method to evaluate design improvements and
determine margin to requirements
Solution:
• Use MEOST strategies to evaluate design improvements
MEOST – Example
Delphi Multec Fuel Injector
Stresses:
� High Pressure: The design limit for purposes of this test was selected
to be the customer requirement: 250 bar. The destruct limit is 600 bar
(based on material properties).
Test Plan:
1. Test 6 pcs each of two design alternatives (“Proto A” and “Proto B”)
2. Run operating rectangle. 10M Cycles between 3 and 250 bar.
3. Increase high pressure in (7) Over-Stress steps of 20% (50 bar and 2M
cycles)
4. Continue to increase part stresses until failure or until all units reach
destruct limit (7 steps).
MEOST – Example
Delphi Multec Fuel Injector
MEOST – Example
Delphi Multec Fuel Injector
“Proto A”
“Proto B”
1.Compress time
2.Increase the stresses that generate failures (accelerated stress testing)
� Elevated (Constant) Stress Testing
� Multiple Stress Level Testing (MEOST)
� Qualitative Methods
Problem: Available Test Time < Expected Lifetime
Solution::
HALT - Highly Accelerated Life Test
• Combined Temperature Cycling, Vibration, and Power Cycling (most
common)
• HALT is performed in specially designed chambers.
• 6 DOF “Repetitive shock” type of vibration
• The HALT test can be carried out in one or two days
• Most often employed with electronic assemblies
The principal idea in HALT is to find the weak links as quickly as possible and then fix
them. After improving one weak link, the next weak link then is found and improved
and so on until there are no weak links left that would cause field failures
HALT is not intended for a quantitative reliability demonstration
Thank you!
John Kreucher
Staff Validation & Reliability Engineer
Delphi Powertrain Systems Division
810-577-9344
