Inverter Reliability Workshop
Accelerated Life Testing
Rob SorensenSandia National [email protected]
Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000This work was performed under funding from the DOE Solar Energy Technologies Program.
Inverter Reliability Workshop
• No PV specific industry standard exists• HALT testing is spotty; independently applied• Separate needs identified for residential and commercial scale inverters • Failure modes identified but not in a uniform program applicable across the
industry • System predictive models will require inputs for inverters
Accelerated Aging for Inverters
Inverter Reliability Workshop
Laboratory testing provides vital information for PV system reliability
Field Data (O&M, Failures, …)
Accelerated Testing / Lab Tests
System performance model must include wear out (end of life) information
Accelerated Aging of Tape Joint – Thermal
Cycling
ALT
Acceleration Factors
Lin k to Performance
Inverter Reliability Workshop
What? Component life tests High stresses
• Single or combined• Activate “appropriate” failure modes• Measureable
Failure analysisWhy? Time Full system is expensive and complicated
What is ALT & why?
Inverter Reliability Workshop
‘Accelerated Aging’
5
Tem
pera
ture
Reaction Coordinate
25°C, 45 days
40°C, 30 days
100°C, 12 min
500°C, 5 min
Inverter Reliability Workshop
6
High T data are extrapolated to “use” conditions (room temperature)
Inverter Reliability Workshop
7
How you extrapolate can influence lifetime predictions.
~50% in ~ 1000 years
~50% in ~ 100 years
~50% in ~ 20 years
Inverter Reliability Workshop
Two approaches to accelerated testing are used throughout industry
Qualitative Accelerated Tests• HALT tests• HAST tests• HASS tests
Quantitative Accelerated Life Tests• Controlled application of accelerated
stress• Produces acceleration factors (AF) Usage rate acceleration
(Time compression) Overstress acceleration
Small sample sizeSevere level of stress
Increase reliability (product improvement)Qualify new designsDesign quantitative ALT
Reliability under normal use conditions
Used to determine TTFDetermine reliability
Long TimeNeed degradation / failure mechanisms
Inverter Reliability Workshop
The Goal of an ALT program is to produce acceleration factors
Often empirical correlations
Limited root-cause analyses
++
−=
obVabVa
n
oRHRH
T1
oT1
kaE
expAF
=
testMTTFMTTFAF field
Time-to-Fail40
1
510
25
50
75
9095
99
50 80 100Cu
mul
ativ
eFa
ilure
Perc
entil
e
MTTF
Inverter Reliability Workshop
Empirical relationships may not cut it!
10080604020
exp [B(T + RH)](RH) exp (E /kT)a
n
exp [E /kT + B(RH) ]a2
exp [E /kT + B/(RH)]a
exp [A/kT + B(RH)/kT + C(RH)]
RH (%)
AF
0
1
102
104
106
Calibration data collected here
Atmospheric Corrosion of Micrelectronics
Five accepted environmental models.All agree at 85% RH (ALT conditions).Orders of magnitude difference at 30% RH (use conditions).
ALT must capture valid degradation / failure mechanisms
Inverter Reliability Workshop
Issues with ALT
Unknown failure mechanisms Unknown / variable use environment Changing mechanisms as function
of environmental stress Difficult to control and
characterize defects Long duration experiments Evolving / improving technology
Inverter Reliability Workshop
What are the likely stresses that lead to Inverter Failure?
Voltage Temperature Thermal cycling Thermal Shock Vibration Mechanical Shock Humidity ??? ??? ???
Heat
Light Moisture
Other
Inverter Reliability Workshop
How do we apply ALT to predicting end-of-life (wear out)?
Determine Failure Mechanisms (field exposures)
Understands stresses that activate the failure mechanisms
Devise ALT conditions that accelerate degradation / failure
Perform ALT (range of conditions)
Determine acceleration factors
Qualify Product
Improve Product Reliability
Validate New Designs
Estimate or Predict Reliabilty
Inverter Reliability Workshop
Example: Al bondpad corrosion: corrosion requires moisture and contamination & is accelerated by temperature.
14
0
500
1000
1500
2000
2500
3000
0 1000 2000 3000
Res
ista
nce
(m-o
hm)
Exposure Time (hrs)
1ug/cm2, 30C, 80% RHPEM 402
d1-w 1
d1-w 3
d1-w 4
d1-w 11
d1-w 12
d1-w 14
d1-w 15
d1-w 22
Three environmental variables (T, RH, [Cl])
No Failures!
Inverter Reliability Workshop
Increasing contaminant level causes failure.
15
0
500
1000
1500
2000
2500
3000
0 50 100 150 200
Res
ista
nce
(m-o
hm)
Exposure Time (hrs)
20 ug/cm2, 30C, 80% RHPEM 414
d1-w 1
d1-w 3
d1-w 4
d1-w 11
d1-w 12
d1-w 14
d1-w 15
d1-w 22
0
500
1000
1500
2000
2500
3000
0 1 2 3
Res
ista
nce
(m-o
hm)
Exposure Time (hrs)
43 ug/cm3, 30C, 80% RHPEM 416
d1-w 1
d1-w 3
d1-w 4
d1-w 11
d1-w 12
d1-w 14
d1-w 15
d1-w 22
20X chloride
40X chloride
Distribution of failure timesNot all failedClear effect of [Cl]
Inverter Reliability Workshop
Statistical treatment (life-data analysis) provides a means of analyzing the bondpad data
16
ReliaSoft Weibull++ 7 - www.ReliaSoft.comReliability vs Time Plot
β=0.2360, η=943.6290, ρ=0.8798
Time, (t)
Relia
bility
, R(t)
=1-F(
t)
0.000 1000.000200.000 400.000 600.000 800.0000.000
1.000
0.200
0.400
0.600
0.800
x 4
x 3
x 2
x 3
Reliability
Data 1Weibull-2PRRX SRM MED FMF=16/S=15
Data PointsReliability Line
Rob SorensenSandia National Labs5/28/201010:46:25 AM
10 ug/cm2, 30C, 30% RH
ReliaSoft Weibull++ 7 - www.ReliaSoft.comReliability vs Time Plot
β=0.7073, η=5.2052, ρ=0.9889
Time, (t)
Relia
bility
, R(t)
=1-F(
t)
0.000 90.00018.000 36.000 54.000 72.0000.000
1.000
0.200
0.400
0.600
0.800
x 2
Reliability
Data 1Weibull-2PRRX SRM MED FMF=14/S=0
Data PointsReliability Line
Rob SorensenSandia National Labs5/28/201010:50:59 AM
20 ug/cm2, 30C, 80% RH
ReliaSoft Weibull++ 7 - www.ReliaSoft.comReliability vs Time Plot
β=2.0355, η=0.6531, γ=0.3818, ρ=0.9576
Time, (t)
Relia
bility
, R(t)
=1-F(
t)
0.000 3.0000.600 1.200 1.800 2.4000.000
1.000
0.200
0.400
0.600
0.800
x 2
x 2
x 3
x 2
Reliability
Data 1Weibull-3PRRX SRM MED FMF=13/S=0
Data PointsReliability Line
Rob SorensenSandia National Labs5/28/201010:57:34 AM
40 ug/cm2, 30C, 80%RH
• Provides distributions• Includes suspension results• Basis for models {Pfail = f(T, RH, [Cl])}
=
testMTTFMTTFAF field
Inverter Reliability Workshop
Bondpad corrosion is modeled as an additional series resistor
Voltage Comparator Circuit Prediction of the effect of corrosion on
LM185 reliability as f(storage location)
The distributed wirebond property (probability of failure) is input into an electrical system model & other component outcomes (reliability, performance threshold) can be determined
y p
0.97
0.975
0.98
0.985
0.99
0.995
1
0 20 40 60 80 100Time (years)
Desert
Gulf Coast
Arctic
Inverter Reliability Workshop
Laboratory testing provides vital information for PV system reliability
Field Data (O&M, Failures, …)
Accelerated Testing / Lab Tests
System performance model must include wear out (end of life) information
Accelerated Aging of Tape Joint – Thermal
Cycling
ALT
Acceleration Factors
Lin k to Performance
Inverter Reliability Workshop
Example: Conductive metal foil tape (Module)
))(028.0(10 α+= tR
Generate ALT data
Develop “acceleration factors”
Module
Load
RIE ×=RIVIP ×=×= 2
Mean
+2σ
-2σ
Determine performance effect
Apply acceleration factors to field
Inverter Reliability Workshop
Use the ALT data to predict long-term performance degradation (wear-out???)
20
Top Related