automating lifetime simulation of power pcbs
DESCRIPTION
Methodology for assessing reliability of power electroncisTRANSCRIPT
© 2004 - 2007© 2004 - 2010© 2004 – 2010
Automating Lifetime Simulation of Power PCBs
ECPE WorkshopNovember 22, 2012
Greg Caswell and Craig Hillman
DfR Solutions, LLC
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o Introduction
o Power PCB Applications
o Common Issues
o Lifetime Expectations
o Failure Mechanisms
o Virtual Qualification Approach
o Sherlock Solution
Agenda
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Power Modules Are Used in Several Market Segments
Thermoelectric Modules
Voltage Power Modules
Solar Power Modules
Automotive Power Modules
200W Power Amp
IGBT
Switching Power Supply
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o High Temperature Environments
o Possible Vibration and Shock Environments
o Temperature and Power Cycling Environments
o Very High Current Flows and Thermal Transfer Requirements
o A variety of materials forming the product
o Substrate tiles bonded to copper baseplate
What Do They All have in Common?
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o IGBT – Rail application – 30 years (Each module 100FIT)
o Power Module – Automotive Application – 20 years
o 10W/cm2
o DBC Substrate bonded to heatsink
o Vibration, shock, humidity, salt spray
o Cost
o Solar Power Inverters-25 years
Example Life Expectancies
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Semicron Thermal Module
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o Thermo-mechanical fatigue induced failureso CTE mismatcho Temperature swings
o Bond Wire Fatigueo Shear Stresses between bond pad and wireo Repeated flexure of the wireo Lift off (fast temperature cycling effect)o Heel Cracking
o Die Attach Fatigueo Solder Fatigue
o Voids
o Device Burn Outo Automotive- degradation of power
o Solder Fatigueo Bond wire failure (lift off due to fast temperature cycling)
o Structural Integrity – ceramic substrate to heat sink in thermal cyclingo IGBTs – solder joint fatigue, wirebond liftoff, substrate fracture, conductor
delamination
Failure Mechanisms
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Bond Wire Fatigue Due to Thermal Effects
Bredtmann, et al, “Options for Electric Power
Steering Modules a Reliability Challenge.”
Automotive Power Electronics, September 2007
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o After 100 cycles of -55 to 200C – DBC Delamination
o By 1000 cycles there were cracks in AlN substrate and extensive solder joint failures
Example of Substrate Delamination
Scofield, Richmond and Leslie, ”Performance and Reliability Characteristics of
1200V,100A 200C Half Bridge SiC MOSFET-JBS Diode Power Modules,”
IMAPS -International Conference on High Temperature Electronics
May 2010
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Failure Modes- Solder and Silicon Cracking
Mitsubishi, “Power Module Reliability”
Cracks between DBC Substrate and also between silicon die and bond wire
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o Examples of wire bond fatigue cracking and also wire bond lift off
Failure Modes - Wire Bond Cracking and Lift Off
Dynex – AN5945 – IGBT Module Reliability
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o The stress conditions in the chart are for a railroad braking application
Typical Mission Profile
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o Typically, extensive qualification testing is performed to ascertain the reliability of the power module as shown
IGBT Qualification Tests-Environmental
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o Power module industry believes copper wire is more robust than aluminumo Changes being implemented for electric drivetrain
o Part of improvement is believed to be due to reduced temperature variation from improved thermal conductivity
o Part of improvement could be due to recrystallizationo Can result in self-healing
o Part of improvement could be more robust fatigue behavior
Copper Wire and Temperature Cycling
D. Siepe, CIPS 2010
N. Tanabe, Journal de Physique IV, 1995
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o Copper clearly superior
Aluminum vs. Copper – Temperature Cycling
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100
106 108107 109
N. Tanabe, Journal de Physique IV, 1995J. Bielen, EuroSime, 2006
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Thermal Aging of Cu Wire Bonds vs. Gold
J. Onuki, M. Koizumi, I. Araki. IEEE Trans. On Comp. Hybrids & Manfg. Tech.
12 (1987) 550
a. b.
Cu
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o Cu is comparable in cost to aluminum but less proven –used on low cost products (not those where the cost of the IC is much greater than the package).
o Cu bonding is slower (5 wires/sec) so that adds process cost if high I/O
o Pd coating helps but adds cost
Points
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o Palladium (Pd) coating creates galvanic couple with copper
o Studies have demonstrated thinning or loss of Pd coating during bonding
o Uncertain if JEDEC test with acceleration factor based on Peck’s equation (based on aluminum/gold galvanic couple) is still valid
o Push out of aluminum pad
o Could result in subsurface cracking (metal migration?)
o Uncertain if existing JEDECtemp cycling test is sufficient todrive crack growth
Major concerns identified by DfR
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o Failure mechanisms
o CTE mismatch resulting in plastic strain
o Thermo-mechanical fatigue as a result of temperature cycling
o Coarsening
Die Attach Fatigue
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Typical Thermal Stress Failures in a Die-Substrate Assembly
Die-Substrate Assembly
Chip, E1,α1
Substrate, E2,α2
Adhesive, E0, α0
Crack at the chip’s corner is due to the
interfacial stresses
Crack at the chip’s surface in its mid-portion
is due to the normal stresses in the chip
Crack/delamination at the
adherend/adhesive interface (adhesive
failure of the bonding material)
Is due to the interfacial stresses
Crack in the body of the adhesive (cohesive failure)
is due to the interfacial stresses
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Typical Failure Modes in Die-Substrate and Similar Assemblies
� Typical failure modes in die-substrate assemblies are:
1) adherend (die or substrate) failure: a silicon die can fracture in itsmidportion or at its corner located at the interface;
2) cohesive failure of the bonding material (i.e., failure of the die-attachmaterial); and
3) adhesive failure of the bonding material (i.e., failure at theadherend/adhesive interface).
� An adhesive failure is not expected to occur in a properly fabricatedjoint. If such a failure takes place, it usually occurs at a very low loadlevel, at the product development stage, and should be regarded as amanufacturing or a quality control failure, rather than a material’s or astructural one.
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Die Attach Solder Reliability
Marie Curie ECON2 2008
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Sherlock
o User Friendly
o Quick
o Flexible
o Intuitive
o Reliable
o One of a Kind
o State of the Art
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Why Sherlock
o Mil-HBK-217 actuarial in nature
o Physics based algorithms to time consuming
o Need to shorten NPI cycles and reduce costs
o Increased computing power
o Better way to communicate
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PoF: The Complexity Roadblock
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Traditional Iterative NPI Cycle
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NPI Cycle Using PoF Modeling
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Why DfA? Total Costs are Determined During Design
95% of the O&S Cost Drivers are Based on Decisions Made during Design.
Source: Architectural Design for Reliability, R. Cranwell and R. Hunter, Sandia Labs, 1997
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Concurrent Engineering
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Introduction
o The foundation of a reliable product is a robust designo Provides margin
o Mitigates risk from defects
o Satisfies the customer
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Intuitive
o Easy-to-locate commands
o Industry terminology (parts list, stackup, pick & place, etc.)
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Reliability Goals
o Compatible with wide variety of reliability metrics
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Ambient Environment
o Handles very complex environments
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Input Design Files
o Takes standard output files (Gerber / ODB)
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Inputs: Parts List
o Color coding of data origino Minimizes data entry through intelligent parsing and
embedded package and material databases
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Part Database Manager
o Enables user to rapidly build their own internal parts database
o Enables user to use both manufacturer and internal part numbers
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Inputs: Stackup
o Automatically generates stackup and copper percent (%)
o Embedded database with almost 400 laminate materials with 48 different properties
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Results: Automated Mesh Generation
o Identifies optimum mesh density based on board size
o Expert user no longer required; model time reduced by 90%
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o Uses embedded FEA engine to compute board deflection and strain cause by ICT fixture
ICT Module (optional)
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o Uses ODB++ data including net list to create board level DFMEA
o Includes customizable spreadsheets for export
DFMEA Module (optional
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Results: Five Different Outputs
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o Comprehensive report generated in PDF format
o Key summary points
o Detailed inputs andfindings
o User control over contents
o 50-100 page professionally formatteddocument
Automated Report Generation
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o Sherlock performs a comprehensive assessment of potential wearout mechanisms from a variety of environmentso Elevated Temperature
o Thermal Cycling
o Random Vibration
o Sinusoidal (Harmonic) Vibration
o Mechanical Shock
o Only software on the market to provide a complete life-cycle predictiono Allows the user to incorporate traditional empirical prediction
as necessary
Unmatched
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Summary - What is Physics of Failure (PoF)?
o Common Definition: o The process of using modeling and simulation based on the
fundamentals of physical science (physics, chemistry, material science, mechanics, etc.) to predict reliability and prevent failures
o Mechanisms that can be modeled include fatigue, creep, diffusion, etc.
o The foundation of a reliable product is a robust designo Provides margino Mitigates risk from defectso Satisfies the customer
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Thank You!Greg Caswell
Sr. Member of the Technical Staff
DfR Solutions