Overcoming 25 year life reliability challenges in solar electronics
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© 2004 - 2007 © 2004 - 2010 © 2004 – 2010 Overcoming 25 Year Life Reliability Challenges in Solar Electronics DfR Solutions Webinar September 27, 2012
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Reliability issues associated with achieving a 25 year warranty
Transcript of Overcoming 25 year life reliability challenges in solar electronics
© 2004 - 2007 © 2004 - 2010 © 2004 – 2010
Overcoming 25 Year Life Reliability Challenges in Solar Electronics
DfR Solutions Webinar
September 27, 2012
© 2004 - 2007 © 2004 - 2010
Solar Panel “Scams” in the News… http://www.bbb.org/blog/2012/06/dont-fall-for-a-solar-paneling-scam-this-summer/
© 2004 - 2007 © 2004 - 2010
Leading Causes of “Hard” Photovoltaic (PV) System Failures
o Central Inverter: 37%
from 2009 Sandia
Study
o IGBT most common
component
o 3 basic fail categories
o Manufacturing Quality
o Inadequate Design
o Defective Electronic
Components
J. Granata, Sandia; 2009 PV
Reliability Conference
IGBT = insulated gate bipolar transistor
© 2004 - 2007 © 2004 - 2010
Leading Causes of “Hard” PV System Failures
o Central Inverter: 51%
from Sun Edison 2008-
2010 study
o Communications: 11%
o Weather Station: 7%
“Owner/Operator Perspective on Reliability Customer Needs and
Field Data”, Sandia National Laboratories, Utility-Scale Grid-Tied
PV Inverter Reliability Workshop, January 2011.
© 2004 - 2007 © 2004 - 2010
Leading Causes of Calls for PV Inverter Failures
o “Other” is largest
o Control software:
16%
o Printed Circuit Board:
11%
© 2004 - 2007 © 2004 - 2010
Leading Root Causes of “Hard” PV Inverter Failures
o Parts & Materials:
57%
o Software: 16%
© 2004 - 2007 © 2004 - 2010
Reliability by Inverter Type
© 2004 - 2007 © 2004 - 2010
PV System Reliability & Availability
o Every alert must be
addressed
o Most software &
communication issues
can be handled quickly
o Hardware takes longer
to resolve
o Potentially easier to
improve availability
by focusing on
software &
communication!
o Tend to be hardware
biased!
© 2004 - 2007 © 2004 - 2010
PV System Failures: Utility Perspective
o Leading failure causes in US:
o Poor Design
o Poor Installation
o Code Infraction
o Ambiguous Instructions
o Lack of information and documentation
o Leading failure causes in developing countries:
o Selling PV systems to users and financing them privately or by soft loans
o Encouraged purchase of cheap, poor quality components and minimal maintenance
o Led to early failures and poor long term sustainability
Evaluation of the PREP Component : PV
Systems for Rural Electrification in Kiribati &
Tuvalu (7 ACP RPR 175)
DESIGN REVIEW AND APPROVAL OF GRID-
TIED PHOTOVOLTAIC SYSTEMS
© 2004 - 2007 © 2004 - 2010
Inverter Overview
o Inverters perform two key functions
o Converts the direct current (DC) coming from the
panels to the alternating current (AC) used by the
electric grid
o Perform algorithms to maximize the power
produced by the system.
© 2004 - 2007 © 2004 - 2010
Types of PV Inverters
Graph courtesy of Doron Shmilovitz, Tel Aviv University, Israel
© 2004 - 2007 © 2004 - 2010
Central Inverters
o PV modules feed into a central dc-dc converter stage that
has:
o Maximum power point tracking (MPPT)
o DC-AC inverter to connect to the utility grid.
o Can’t maximize energy extraction from all modules
© 2004 - 2007 © 2004 - 2010
Micro-Inverters vs Central Inverters
o Better Reliability & Availability o Lack of single failure point
o Longer warranty: 15-25 years versus 5-10 years
o Lower DC Voltages, less vulnerable to arcing
o Optimized Maximum Power Point Tracking (MPPT) per module
o PV Module level real-time monitoring
Images courtesy of Paul Parker, SolarBridge
© 2004 - 2007 © 2004 - 2010
Micro-Inverters
o The electronic components used in a micro-inverter are commercial off-the-shelf (COTS)
o Parts designed for consumer electronics but need to survive 25 years in solar installations
o Outdoor/Partially Protected & Temp Not Controlled
© 2004 - 2007 © 2004 - 2010
Distributed Inverters
o Similar advantages as Micro
o Designed to work with power optimizers
o Standard 10-12 year warranty, some extended to 20-25 years
o Built-in module-level monitoring receiver with communication to internet via broadband or wireless
o Because MPPT & voltage management are handled separately for each module by the power optimizer, the inverter is only responsible for DC to AC inversion. So potentially less complicated, expensive & more reliable
© 2004 - 2007 © 2004 - 2010
Micro vs Distributed Inverters
o From a system reliability and maintainability perspective,
both offer advantages over Central inverters.
o All inverter types face the same environmental and
electronics failure modes.
o Both regarded as more cost effective for commercial
applications vs utilities
o Both add significant software complexity for optimizing and
monitoring functions
© 2004 - 2007 © 2004 - 2010
Micro vs Distributed Distinctions
o Component Selection o Life limits of Electrolytic Caps versus Ceramic Caps
o Use of ASICs to reduce discreet parts count
o Derating o Power optimizers are low voltage devices and more easily highly derated
o Efficiency & Heat Dissipation Differences
o Distributed/string benefits are true only if designed for the specific optimizer o One company doing this as a complete system
o No current standard for DC Optimizers, so other string inverter companies may be hesitant to design a product around a specific vendors’ optimizer.
© 2004 - 2007 © 2004 - 2010
Micro vs Distributed Distinctions
o Ceramic versus electrolytic capacitors
o DC/DC converters rely on ceramic capacitors which have a low, fixed rate of aging. o Possible due to the relatively high
switching frequency used in converters.
o Micro inverters require large input capacitance due to the grid low frequency. o In some cases, this is implemented
with electrolytic capacitors which have a significantly shorter lifetime.
Graphic courtesy of SolarEdge
© 2004 - 2007 © 2004 - 2010
Component Failures
Image Courtesy of SolarBridge
© 2004 - 2007 © 2004 - 2010
Micro vs Distributed Distinctions
o ASICs allow embedding many electronics into the chip,
thereby
o Reduces number of discreet components and potential points
of failure.
o Reduced component count on the circuit board also allows
manufacturers of power optimizers to decrease the overall
size of the product and raise MTBF
© 2004 - 2007 © 2004 - 2010
Micro vs Distributed Distinctions
o Derating
o Power optimizers are low voltage devices and more easily
highly derated
o A lower derating factor may shorten the lifetime of the
product due to increased stress under operating
conditions.
© 2004 - 2007 © 2004 - 2010
Micro vs Distributed Distinctions
o Efficiency & Heat Dissipation Differences
o Micro-inverters have lower efficiencies than power
optimizers. The highest known efficiency of micro-
inverter brands is 96%, meaning 4% heat
dissipation to the module.
© 2004 - 2007 © 2004 - 2010
Micro vs Distributed Distinctions
o Distributed/string benefits are true only if
designed for the specific optimizer
o One company doing this as a complete system
o No current standard for DC Optimizers, so other string
inverter companies may be hesitant to design a product
around a specific vendors’ optimizer.
© 2004 - 2007 © 2004 - 2010
Inverter Field Failure Mechanisms
o Solder joint fatigue failure
o Plated through hole fatigue failure
o Conductive anodic filament formation (CAF)
o Shock or Vibration (shipping and in use)
o Component wear out
o Potting Induced Failure
© 2004 - 2007 © 2004 - 2010
o Solder joints “wear out” or fatigue and fail under
the long term influence of temperature cycling
and mechanical stresses.
Solder Fatigue
© 2004 - 2007 © 2004 - 2010
Plated Through Hole Fatigue o When a printed circuit board experiences temperature
cycling, expansion/contraction in the z-direction is much
higher than that in the x-y plane
o High stress can build up in the copper via barrels resulting
in cracking near the center of the barrel as shown in the
cross section photos below.
© 2004 - 2007 © 2004 - 2010
Conductive Anodic Filament Formation
(CAF) o CAF formation is a risk when Plated Through Hole (PTH) vias
are so close together that damage from drilling can open up a
pathway between vias.
o Copper from the via can migrate along the pathway and
eventually cause shorting.
© 2004 - 2007 © 2004 - 2010 28
Failure after Exposure to Vibration
o Mechanical shock and vibration also leads to solder joint
failures
o Can occur during transportation, installation or use
© 2004 - 2007 © 2004 - 2010
Potting Electronic Assemblies
o Potting is the process of filling an
electronic assembly with a resin
compound
o Provides resistance to shock and
vibration, and excludes moisture
and corrosives.
© 2004 - 2007 © 2004 - 2010
Printed Circuit Board Warpage due to
Potting Shrinkage
© 2004 - 2007 © 2004 - 2010
o The use of underfills, potting compounds and thick
conformal coatings greatly influences the failure behavior
under temperature cycling
o Any time a material goes through its glass transition (Tg)
temperature problems tend to occur
o Underfills designed for enhancing shock robustness do not enhance
thermal cycling robustness
o Potting materials can cause PCB warpage and tensile stresses on
electronic packages that greatly reduce time to failure
Potting Rules of Thumb
© 2004 - 2007 © 2004 - 2010
Some Questions You Should Ask Your Inverter Supplier
o How have you evaluated reliability?
o Not MTBF, certifications or specifications but evaluation under
stress conditions to failure
o What is your field failure history & repair rate?
o What fails & why?
o How long and how many units in the field?
o Who installs them?
o What software monitoring options are available?
© 2004 - 2007 © 2004 - 2010 © 2004 – 2010
An Effective Means to Model the
Life of Inverter Electronics
© 2004 - 2007 © 2004 - 2010
Micro-Inverter Requirements
o The electronic components used in a micro-inverter
are off-the-shelf which means they were designed
for consumer electronics.
o How will such electronic assemblies survive the
demands of the solar industry?
Consumer Electronics Micro-Inverter Electronics
Expected Life 5-7 years 20-25 years
User Environment Indoor/Protected Outdoor/Partially Protected
Temp Controlled Temp Not Controlled
© 2004 - 2007 © 2004 - 2010
What can be done?
o How can a micro-inverter supplier design the product to meet
the requirements AND convince the customer of this?
o This talk will discuss a novel new method to model the
reliability of an electronic assembly in a variety of conditions
based on the design (before building anything).
o Design for Reliability (DfR) concepts and Physics of Failure
(PoF) are used. .
o A comprehensive software package was developed to
simplify this modeling, thus making it available to design
engineers.
© 2004 - 2007 © 2004 - 2010
Design for Reliability (DfR)
o DfR: A process for ensuring the reliability of a
product or system during the design stage before
physical prototype
o Reliability: The measure of a product’s ability to
o …perform the specified function
o …at the customer (with their use environment)
o …over the desired lifetime
© 2004 - 2007 © 2004 - 2010
DfR & Physics of Failure (PoF)
o Due to some of the limitations of classic DfR, there has been an increasing interest in PoF (aka, Reliability Physics) – to improve on DfR techniques.
o PoF Definition: The use of science (physics, chemistry, etc.) to capture an understanding of failure mechanisms and evaluate useful life under actual operating conditions
© 2004 - 2007 © 2004 - 2010
Micro-Inverter Environment
o Extreme hot and cold locations (AZ to AK)
o Possible exposure to moisture/humidity
o Large diurnal thermal cycle events (daily)
o Largest temp swings occur in desert locations where it
can reach 64C in the direct sun down to 23C at night
(Δ41C)
© 2004 - 2007 © 2004 - 2010
NREL – Solar Panel Data
o Diurnal Cycles for Each Month
© 2004 - 2007 © 2004 - 2010
Possible Inverter Failure Mechanisms
o Solder joint fatigue failure
o Plated through hole fatigue failure
o Conductive anodic filament formation (CAF)
o Shock or Vibration (during shipping)
o Component wear out.
© 2004 - 2007 © 2004 - 2010
Is There a Method to Model these Failure
Mechanisms?
o Yes
o Algorithms exist to estimate the failure rate from
solder joint fatigue for different types of components.
o IPC TR-579 models PTH reliability
o Risk for CAF can be determined
o Finite Element Analysis can be used for Shock &
Vibration risk.
o MTBF calculations can be performed to estimate
component failure rates.
© 2004 - 2007 © 2004 - 2010
Leverage in Product Design
70% of a Product’s Total Cost is Committed by Design
http://www.ami.ac.uk/courses/topics/0248_dfx/index.html
© 2004 - 2007 © 2004 - 2010
Why is modeling reliability early
important?
Architectural Design for Reliability, R. Cranwell and R. Hunter, Sandia Labs, 1997
© 2004 - 2007 © 2004 - 2010
Earlier is Cheaper
Reduce Costs by Improving
Reliability Upfront
© 2004 - 2007 © 2004 - 2010
What is the cost impact of poor reliability?
Aberdeen Group, Printed Circuit Board Design Integrity: The Key to Successful PCB Development, 2007
http://new.marketwire.com/2.0/rel.jsp?id=730231
© 2004 - 2007 © 2004 - 2010
Solder Joint (SJ) Wearout
o Elimination of leaded devices
o Provides lower RC and higher package densities
o Reduces compliance
Cycles to failure
-40 to 125C QFP: >10,000 BGA: 3,000 to 8,000
QFN: 1,000 to 3,000 CSP / Flip Chip: <1,000
© 2004 - 2007 © 2004 - 2010
The Newest DfR Tool - Sherlock
o Sherlock – a new tool that models all the circuit card
assemblies and provides predicted life curves from
many common failure mechanisms.
o It is a Semi-Automated CAE knowledge based
program
o The Semi-Automated Features simplify model creation
and analysis
o Eliminates the long, complicated, model creation process and
the need for a PhD level expert in PoF, FEA and CFD
numerical modeling.
© 2004 - 2007 © 2004 - 2010
Software Coverage
o This software modeling tool predicts failures from
o Solder joint wear-out from thermal cycling (SAC305 or
eutectic SnPb)
o Plated through hole fatigue
o Conductive anodic filament formation
o 217 MTBF calculations are also generated
o In addition the software uses FEA to determine
o Board deflection and SJ failure from mechanical vibration
o The natural frequencies for the board based on the mount
points.
o Board deflection due to shock events
© 2004 - 2007 © 2004 - 2010
o ODB files are preferred since they contain all the data
necessary for modeling
o Component details and placement
o PCB outline & stack-up
o Drill hole file with mount points
o Metal layers, silkscreen, solder mask layers
Data Import
Gerber Data can
also be imported
© 2004 - 2007 © 2004 - 2010
PCB Details Required for Modeling
© 2004 - 2007 © 2004 - 2010
Establish Part Parameters
o Components identified along with packaging properties.
o Minimizes data entry through intelligent parsing and embedded package and material databases
© 2004 - 2007 © 2004 - 2010
Defining The Durability & Reliability Objectives
o Define the Expected Service Life
o Sets The Analysis Range And The Scale For Reliability Over Time Plots
o Life Cycle Phases
o Shows relative time spent at each phase
© 2004 - 2007 © 2004 - 2010
Identify Field Environment o Approach 1: Use of industry/military specifications
o MIL-STD-810,
o MIL-HDBK-310,
o SAE J1211,
o IPC-SM-785,
o Telcordia GR3108,
o IEC 60721-3, etc. o Advantages
o Sometimes very comprehensive
o Agreement throughout the industry o Disadvantages
o Most more than 20 years old
o Always less or greater than actual (by how much, unknown)
© 2004 - 2007 © 2004 - 2010
Field Environment (cont.)
o Approach 2: Based on actual measurements of similar products in similar environments
o Determine average and realistic worst-case
o Identify all failure-inducing loads
o Include all environments
o Manufacturing
o Transportation
o Storage
o Field
Container and Ambient Temperature
15.0
25.0
35.0
45.0
55.0
65.0
75.0
0 50 100 150 200 250 300 350 400 450
Hours
Te
mp
era
ture
(°C
)
Container Temp (°C)
Outdoor Temp (°C)
© 2004 - 2007 © 2004 - 2010
Input Field (or Test) Environment
Thermal Cycle
Profile
Vibration Profile
Shock Profile.
Handles very complex environments
© 2004 - 2007 © 2004 - 2010
Thermal Cycle Modeling
o The thermal cycle conditions the product will experience can be
modeled (Minor’s rule applied for multiple temperatures).
o Powered components can be assigned an added value above
ambient temperature.
Example of Diurnal TC Conditions in a hot climate
© 2004 - 2007 © 2004 - 2010
TC Fatigue Analysis – Example Plot
© 2004 - 2007 © 2004 - 2010
Highest Risk Components
o Large Resistors provide the weakest solder joints in this example.
© 2004 - 2007 © 2004 - 2010
Plated Through-Hole Reliability Modeling
When a PCB experiences thermal cycling,
the expansion/ contraction in the z-
direction is much higher than that in the x-y
plane.
The glass fibers constrain the board in the
x-y plane but not through the thickness.
As a result, a great deal of stress can be
built up in the copper via barrels resulting
in eventual cracking near the center of the
barrel as shown in the cross section photos.
© 2004 - 2007 © 2004 - 2010
PTH Fatigue Results - Example
© 2004 - 2007 © 2004 - 2010
Combined (SJ & PTH) Lifetime Prediction
o Combines analysis results into overall failure prediction curve
PTH
Solder Joint
Combined
© 2004 - 2007 © 2004 - 2010
Risk for Conductive Anodic Filament Formation
(CAF)
o CAF formation becomes a risk when plated through hole vias
are so close together that damage from drilling can open up a
pathway between vias.
o Copper from the via can migrate along the pathway and
eventually cause shorting.
© 2004 - 2007 © 2004 - 2010
CAF Analysis
o The primary variables that effect the probability of
CAF formation are:
o Distance between vias
o Damage during drilling process
o Temperature and humidity conditions
o Voltage differential between vias
o The analysis takes into account the first two variables
only (measures distance between all PTH pairs).
o Vias identified as being too close are flagged.
© 2004 - 2007 © 2004 - 2010
CAF Analysis
o Software will flag vias at high risk for CAF formation
© 2004 - 2007 © 2004 - 2010
Finite Element Analysis
o PCBA Example with Mesh Outlined
© 2004 - 2007 © 2004 - 2010
Natural Frequencies are Calculated
Select the number of
natural frequencies to
look for within the
desired frequency
range.
Components in high
strain regions are at
risk.
• Move the
components
• Move/add
mounting points.
MPs
© 2004 - 2007 © 2004 - 2010
Vibration Environment
Complex vibration profiles can be
modeled.
• Qualification test parameters
• Shipping/Transportation
• Field conditions
© 2004 - 2007 © 2004 - 2010
Random Vibration Strain
Board strain from
random vibration is
shown (calculated
for x, y, and z
directions)
Vibration set to simulate shipping
© 2004 - 2007 © 2004 - 2010
Vibration Results – Component Breakdown
Components most at risk for vibration fatigue damage are listed first.
© 2004 - 2007 © 2004 - 2010
Software Shock
o Implements Shock based upon a critical board
level strain
o Will not predict how many drops to failure
o Either the design is robust with regards to the
expected shock environment or it is not
o Additional work being initiated to investigate
corner staking patterns and material influences
© 2004 - 2007 © 2004 - 2010
Shock Results - Example
© 2004 - 2007 © 2004 - 2010
Shock Results – Component Breakdown
Components listed in order of maximum strain
experienced.
© 2004 - 2007 © 2004 - 2010
Constant Failure Rate Module – Components (Mil-
HNBK-217F)
This takes into account the failure rate of the
components themselves.
© 2004 - 2007 © 2004 - 2010
Combined Failure Rate is Provided
© 2004 - 2007 © 2004 - 2010
Additional Uses for Modeling
o Use Sherlock to determine thermal cycle test
requirements.
o Use to modify mount point locations
o Use to determine ESS conditions
o Component Replacement
o Determine impact of changing to Pb-free solder
o Determine expected warranty costs
© 2004 - 2007 © 2004 - 2010
Warranty Cost Estimate: Example
•Solder joint fatigue was the dominant
wearout mechanism, where the failure
rate is predicted ~2% after 20 yrs.
•Warranty from SJ failure = failure rate *
motherboard replacement cost * units
sold.
© 2004 - 2007 © 2004 - 2010
Summary
o It is important to eliminate design flaws early in development.
o Micro-Inverters must survive a challenging environment for long periods of time.
o A software tool is now available to model the primary failure mechanisms so that inverter electronics can be made more reliable.
o Sherlock modeling will enable a number of “what if” scenarios. o Changing package types
o Changing location of components
o Changing the mount point locations
o Changing laminate type, etc.
o The software can also be used to determine the TC test conditions that best simulate the field use conditions.
o Micro-Inverter designs can be built with more confidence that they will survive the challenging environments where they are placed.
© 2004 - 2007 © 2004 - 2010
Questions
Thank you for your attention.
Any questions?
