Corrosion and Accelerated Testing

Rob Sorensen 505-844-5558 nrsoren@sandia.gov Sandia National Laboratories

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-AC04-94AL85000.

What is corrosion?

Aqueous (general & localized attack) • electrochemical • oxidation / reduction

Atmospheric • gas-metal reaction (slow) • condensed phase

electrochemical • pollutant gasses (ppt levels of H2S, NO2, Cl2 …)

Environmental degradation of materials

Processing

Use

Corrosion normally occurs due to defects or unexpected environments

Cruise missile fuel line

A plastic encapsulated IC failed after 5 years in dormant storage.

Defects:

Damaged passivation

Au (galvanic couple)

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Several non-traditional corrosion mechanisms exist in microelectronics that involve electrical bias

Electrolytic dissolution

Cathodic corrosion (alkalization)

Electrolytic migration

Conditions required for electrolytic migration

Applied voltage • powered system

Susceptible alloy • Ag used in ground plane • Solder • Copper

Conductive surface • flux residue (activator)

Electrolyte • High humidity Temperature cycling

• Seacoast environment (NaCl)

Corrosive environments can contact metallization features

Breach in hermetic packaging in CHP Use of encapsulants in PEMs with

high water permeability

Specific unintended exposures (e.g., military) high T, RH, [Cl-] possible

Materials Degradation Affects Reliability

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Probability of failure-free performance, item's useful life, or a specified timeframe specified environmental duty-cycle conditions.

Cost Reliability

Lab and Field Data Accelerated Tests

Accelerated testing plays a key role in determining reliability Must be applicable to failure

modes Reproduce field failures

Acceleration factor Range of stresses Long term tests

The inverter is a significant contributor to reliability issues

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Sun Edison – Owner/Operator (A. Golnas, “PV System Reliability: An Operator’s Perspective,” in 38th Photovoltaic Specialists Conference, Austin, TX, Jun. 2012, pp. 1–32.)

EVENTS

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Tucson Electric Springerville Plant. Sandia Study

What? Component life tests High stresses

• Single or combined • Activate “appropriate” failure modes • Measureable

Time compression (cyclic stresses) Failure analysis Why? Time Full system is expensive and complicated

What is ALT & why?

Failure Modes for Crystalline Silicon (John Wolgemuth – BP Solar)

Broken interconnects Broken Cells Corrosion Delamination and/or loss of elastic properties Encapsulant discoloration Solder bond failures Broken glass Hot Spots Ground faults Junction box and module connection failures Structural failures

Would you expect a single test to capture all of the expected failure modes?

It is important to understand the degradation mechanism and select appropriate stress level

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Tem

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Reaction Coordinate

25°C, 45 days

40°C, 30 days

100°C, 12 min

500°C, 5 min

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High T data are extrapolated to “use” conditions (room temperature)

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How you extrapolate can influence lifetime predictions.

~50% in ~ 1000 years

~50% in ~ 100 years

~50% in ~ 20 years

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 size Severe level of stress

Increase reliability (product improvement) Qualify new designs Design quantitative ALT

Reliability under normal use conditions

Used to determine TTF Determine reliability

Long Time Need degradation / failure mechanisms

The Goal of an ALT program is to produce acceleration factors

Often empirical correlations

Limited root-cause analyses

++

−=

obVabVa

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oRHRH

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testMTTFMTTFAF field

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ALT must capture valid degradation / failure mechanisms

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exp [B(T + RH)](RH) exp (E /kT)a

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exp [E /kT + B/(RH)]a

exp [A/kT + B(RH)/kT + C(RH)]

RH (%)

AF

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Calibration data collected here

Atmospheric Corrosion of Micrelectronics

Example:

• Five recognized models for corrosion in micro-electronics

• All agree at 85%RH

• Disagreement at 10%-30% prevent uniform application of either model

Acceleration factors depend on the stress characteristics

Thermal (Arrhenius) • Activation energy • Verify no mechanism change • Bin damage by time at temperature

∆T • Linear (time compressions), • Increased temperature range • Frequency analysis (rainflow counting)

Voltage • Linear (must understand relationship)

Humidity • Tends to be complex (adsorbed water)

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Date and Time (MST)-July

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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

What are the likely stresses that lead to Inverter Failure?

Voltage Temperature Thermal cycling Thermal Shock Vibration Mechanical Shock Humidity Contamination Mechanical Stress ??? ???

Heat

Light Moisture

Other

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 stressses)

Determine acceleration factors

Qualify Product

Improve Product Reliability

Validate New Designs

Estimate or Predict Reliabilty

Analysis of metal foil tape degradation

))(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

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Use the ALT data to predict long-term performance degradation (wear-out???)

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Summary

Accurate prediction of reliability is complex Requires understanding of degradation processes Data Driven

• Field data • Accelerated testing

Effect on performance (what is failure?) Includes uncertainty

Extra Slides

Example: Al bondpad corrosion: corrosion requires moisture and contamination & is accelerated by temperature.

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1ug/cm2, 30C, 80% RHPEM 402

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Three environmental variables (T, RH, [Cl])

No Failures!

Increasing contaminant level causes failure.

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20X chloride

40X chloride

Distribution of failure times Not all failed Clear effect of [Cl]

Statistical treatment (life-data analysis) provides a means of analyzing the bondpad data

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ReliaSoft Weibull++ 7 - www.ReliaSoft.comReliability vs Time Plot

β=0.2360, η=943.6290, ρ=0.8798

Time, (t)

Relia

bility

, R(t)

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t)

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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

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=1-F(

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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(

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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

Bondpad corrosion is modeled as an additional series resistor

Voltage Comparator Circuit Prediction of the effect of corrosion on 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

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Accelerated aging induced capacitor degradation.

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Inverter Temperature Profile

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Solectria Inverter - July

Pre failure signature

Capacitor failed in accelerated testing

Accelerated aging stresses included voltage and temperature

Accelerated Aging • Voltage & Temperature • Capacitor failure • Failure pre-cursor (decreased

capacitance)

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

Field failure data from very large plants indicate correlation between temperature and failure

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Predictor Variables AC_Power Cabinet_Temp Coolant_Temp DC_AC DC_Current DC_Power DC_Voltage Inv_Amb

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Failures concentrated here

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Failed Units

Data from large plant

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1 Failed Inverter 222 Operational Inverters

AC/DC

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10 Failed Inverters 55 Operational Inverters

0 Failed Inverters 20 Operational Inverters

N=308

N=85

Accelerated Aging for Inverter Development • 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

Atmospheric Corrosion

Ceramic flat pack

Au/Al wirebond

Diode

Examples of electrolytic migration

LF amplifier - Puerto Rico • high humidity • seacoast environment (NaCl) • powered system • Ag used in ground plane

Printed wiring boards • cyclic humidity • flux residue (activator) • powered electronics • Sn/Pb solder