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1 © 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 [email protected]

Offshore Wind Turbines Power Electronics Design and ... · Conductive Filament Formation (CFF) • Conductive filament formation (CFF), also referred to as metallic electromigration

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Page 1: Offshore Wind Turbines Power Electronics Design and ... · Conductive Filament Formation (CFF) • Conductive filament formation (CFF), also referred to as metallic electromigration

1

© 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

[email protected]

Page 2: Offshore Wind Turbines Power Electronics Design and ... · Conductive Filament Formation (CFF) • Conductive filament formation (CFF), also referred to as metallic electromigration

2

© 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..

Page 3: Offshore Wind Turbines Power Electronics Design and ... · Conductive Filament Formation (CFF) • Conductive filament formation (CFF), also referred to as metallic electromigration

3

© 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.

Page 4: Offshore Wind Turbines Power Electronics Design and ... · Conductive Filament Formation (CFF) • Conductive filament formation (CFF), also referred to as metallic electromigration

4

© 2014 - University of Maryland

Corrosion in Wind Turbines

Page 5: Offshore Wind Turbines Power Electronics Design and ... · Conductive Filament Formation (CFF) • Conductive filament formation (CFF), also referred to as metallic electromigration

5

© 2014 - University of Maryland

Corrosion Avoidance and Mitigation

Page 6: Offshore Wind Turbines Power Electronics Design and ... · Conductive Filament Formation (CFF) • Conductive filament formation (CFF), also referred to as metallic electromigration

6

© 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

Page 7: Offshore Wind Turbines Power Electronics Design and ... · Conductive Filament Formation (CFF) • Conductive filament formation (CFF), also referred to as metallic electromigration

7

© 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

Page 8: Offshore Wind Turbines Power Electronics Design and ... · Conductive Filament Formation (CFF) • Conductive filament formation (CFF), also referred to as metallic electromigration

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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

Page 9: Offshore Wind Turbines Power Electronics Design and ... · Conductive Filament Formation (CFF) • Conductive filament formation (CFF), also referred to as metallic electromigration

9

© 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.

Page 10: Offshore Wind Turbines Power Electronics Design and ... · Conductive Filament Formation (CFF) • Conductive filament formation (CFF), also referred to as metallic electromigration

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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.

Page 11: Offshore Wind Turbines Power Electronics Design and ... · Conductive Filament Formation (CFF) • Conductive filament formation (CFF), also referred to as metallic electromigration

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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

Page 12: Offshore Wind Turbines Power Electronics Design and ... · Conductive Filament Formation (CFF) • Conductive filament formation (CFF), also referred to as metallic electromigration

12

© 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

Page 13: Offshore Wind Turbines Power Electronics Design and ... · Conductive Filament Formation (CFF) • Conductive filament formation (CFF), also referred to as metallic electromigration

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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

Page 14: Offshore Wind Turbines Power Electronics Design and ... · Conductive Filament Formation (CFF) • Conductive filament formation (CFF), also referred to as metallic electromigration

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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)

Page 15: Offshore Wind Turbines Power Electronics Design and ... · Conductive Filament Formation (CFF) • Conductive filament formation (CFF), also referred to as metallic electromigration

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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

Page 16: Offshore Wind Turbines Power Electronics Design and ... · Conductive Filament Formation (CFF) • Conductive filament formation (CFF), also referred to as metallic electromigration

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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

Page 17: Offshore Wind Turbines Power Electronics Design and ... · Conductive Filament Formation (CFF) • Conductive filament formation (CFF), also referred to as metallic electromigration

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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.

Page 18: Offshore Wind Turbines Power Electronics Design and ... · Conductive Filament Formation (CFF) • Conductive filament formation (CFF), also referred to as metallic electromigration

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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

Page 19: Offshore Wind Turbines Power Electronics Design and ... · Conductive Filament Formation (CFF) • Conductive filament formation (CFF), also referred to as metallic electromigration

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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