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© 2014 - University of Maryland
Offshore Wind Turbines
Power Electronics Design and
Reliability Research
F. P. McCluskey
CALCE/Dept. Of Mechanical Engineering
University of Maryland, College Park, MD
(301) 405-0279
mcclupa@umd.edu
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© 2014 - University of Maryland
Power electronics are an integral part of offshore
wind turbine energy production
University of Maryland research focuses
on the power electronics in the nacelle
where variable frequency converters
modify the AC electric energy created by
the wind-driven generators to high
voltage AC (50 or 60 Hz) or high voltage
DC for transmission to shore and
distribution on the grid..
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© 2014 - University of Maryland
The “Marine Environment”
• The “marine environment” refers to all the physical, chemical, and biological stressors that
would typically be present and acting on wind turbine electronics in an offshore
application.
• The primary concern and the main focus of this study is the effects of the presence of salt
and moisture content, in the form either of sea water, sea air, or salt spray on corrosion of
the electronics. This corrosion can take many forms and occur at many sites in wind
turbine electronics.
• Some of this concern is mitigated by the practice of “normalizing the turbine internal
atmospheric environment” by removing salt from all air inflows and providing a positive
pressure within the turbine itself. The mitigating effect of lower salt and moisture levels
remaining in the air around the electronics after normalization will be addressed.
• There are, however, other physical and electrical stresses related to location offshore that
can cause failures as well. Locating wind turbines offshore exposes them to harsh weather
conditions such as:
– highly variable temperatures
– powerful storms and lightning strikes.
• These can cause failures resulting from solder fatigue due to temperature cycling or due to
the shock or vibration caused by high winds or waves during storms.
• Application life cycle profiles are being generated based on the marine environmental and
operational conditions and extremes.
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© 2014 - University of Maryland
Corrosion in Wind Turbines
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© 2014 - University of Maryland
Corrosion Avoidance and Mitigation
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© 2014 - University of Maryland
Power Electronic System PoF Reliability Assessment
Overall system
Part1 Partn
Est
imat
ion o
f th
e over
all
syst
em
Failure
mechanism1
Part2
Parts arranged in
different configurations
e.g., series, parallel
Failure
mechanism2
Failure
mechanism2
Failure
mechanism2
Failure
mechanismn
Failure
mechanismn
Failure
mechanismn
Failure
mechanism1
ii
fi
ii
f
ii
f rd)r(d)R(
Sub-system1 Sub-system2Sub-systemn
mKAdN
da
Nf = 0.5 (g/2f)c
PoF
mech
anism
iden
tification
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© 2014 - University of Maryland
Steps to Addressing Corrosion Failures
• Design information has been collected for power electronics
used in offshore wind turbines.
• Currently identifying models and mitigation approaches for
key failure mechanisms in power electronics in marine
environments, such as the following:
– Electrochemical Migration on Power/Gate Driver Boards
– Silver Metal Migration
– Conductive Filament Formation in Power Boards
– Corrosion of the Direct Bond Copper (DBC) Substrate
– Corrosion of Copper Leads
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© 2014 - University of Maryland
Electrochemical Migration
Electrochemical migration (ECM) is the growth of conductive metal filaments on or in a printed circuit board (PCB) under the influence of a DC voltage bias [IPC-TR-476A].
Necessary Conditions for ECM
• Electrical carriers (such as ions).
• A medium, usually water, to dissolve the ionic materials and sustain them in their mobile ionic state.
• Electrical potential between the electrodes to establish an ionic current in the liquid medium.
Stages of ECM
• Path formation
• Electrodissolution
• Ionic transport
• Electrodeposition
• Filament growth
Substrate
Anode (Cu): electrodissolution Cathode (Cu): electrodeposition
DC voltage source+ -
Solder alloy
Plating
Ion transport
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© 2014 - University of Maryland
Dendritic Growth
Solder
Exposed
SubstrateSolder Mask
(green area)
Dendritic growth occurs on the top surface of the printed wiring boards between adjacent
isolated conductors. Electrical bias and surface contamination from flux residue or process
residue can contribute to the growth. Surface damage may also play a role.
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© 2014 - University of Maryland
250 μm
Silver Migration on Lead-Free PCB
During THB Testing
Sn-3.5Ag Solder on
Polyimide Substrate with
Immersion Sn Plating
EDS mapping revealed thatsilver migrated between thetwo electrodes. Sn-3.5Agwas the only source ofsilver in this sample.
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© 2014 - University of Maryland
Conductive Filament Formation (CFF)
• Conductive filament formation (CFF),
also referred to as metallic
electromigration or conductive anodic
filament (CAF), is an electrochemical
process which involves the transport
(usually ionic) of a metal across a
nonmetallic medium under the
influence of an applied electric field.
• CFF can cause current leakage,
intermittent electrical shorts and
dielectric breakdown between
conductors in printed wiring boards.
Anode
(Positive)
Cathode
(Negative)
150 µm
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© 2014 - University of Maryland
A. Plated-through hole (PTH) to
PTH
B. Trace to trace (or
plane)
C. Trace/Plane to PTH
Pla
ted
Th
rou
gh
Ho
le
Pla
ted
Th
rou
gh
Ho
le
Typical CFF Paths
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© 2014 - University of Maryland
Formation of Conductive Filaments
Cu
Cu
Cu
CuCu
CuCu++ Cu++
Cu++
Cu++
Oxidation Sites
Reduction Sites
PTH PTH
Epoxy Resin
Epoxy Resin
Glass Fiber
Water Monolayers
Delamination
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© 2014 - University of Maryland
Model for Assessing Time-to-Failure for CFF
Tf = time to failure (hrs) n = geometry; acceleration factor
a = filament formation acceleration factor m = voltage acceleration factor
f = multilayer correction factor M = Moisture absorbed
Leff = effective length between conductors (inches) Mt = Threshold moisture content
V = Voltage (Volts dc)
)(
)1000(
t
m
n
eff
fMMV
LfaT
800
VDC
300
VDCSM/Pb-Sb/No PC
No SM/No PC
Experimental data (300 VDC)
Experimental data (800 VDC)
0 10 30 4020
200
400
600
800
1000
0
1200
1600
Spacing (mils)
Tim
e to
Fai
lure
(h
rs)
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© 2014 - University of Maryland
DBC Corrosion
• Galvanic corrosion between dissimilar metals in contact, such as Al wires
on DBC substrates in power modules
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© 2014 - University of Maryland
Copper Lead Failure
1 mm
• Initial optical inspection of lead cross-section
– New module - no vibration or thermal cycling
– Microcracks visible on edges of lead pads
1 mm
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© 2014 - University of Maryland
Other Potential Corrosion Failures
• Pitting corrosion or crevice corrosion, where moisture and chlorides are
present but there is a localized reduction in the oxygen level leading
these sites to become anodic and initiating corrosion on substrates or
PWB traces;
• Erosion-corrosion, where any salt or sand particulates from the marine
environment can cause wear of metal, or of the native protective oxide
films or protective coatings on the metal, for example, on PWB traces,
leading to corrosion.
• Marine –biological Corrosion in which microbiological organisms can
deposit and exclude oxygen, creating locally corrosive conditions.
Furthermore, deposits of these microbiological organisms can short
traces together and increase resistance.
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© 2014 - University of Maryland
Microbiologically Influenced Corrosion (MIC)
• Initiation and acceleration of corrosion due to the interaction
between microbial activity and construction materials
• Applications
– Offshore construction, piping, ship ballast tanks, sprinkler systems,
water infrastructure
• Example of MIC: Accelerated Low Water Corrosion
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© 2014 - University of Maryland
Conclusion
• We are developing PoF models for failure of offshore
wind turbine electronics in marine environments
• We are developing methods to integrate these failure
models into techniques for system-wide reliability
assessments, based on:
– Fault tree analysis
– Probabilistic physics of failure
– Bayesian approaches
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