Automating lifetime simulation of power PCBs

Automating Lifetime
Simulation of Power PCBs
ECPE Workshop
November 22, 2012
© 2004 -–2200011070
Greg Caswell and Craig Hillman
DfR Solutions, LLC

Agenda
o Introduction
o Power PCB Applications
o Common Issues
o Lifetime Expectations
o Failure Mechanisms
o Virtual Qualification Approach
o Sherlock Solution
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Power Modules Are Used in Several Market Segments
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Switching Power Supply
Thermoelectric Modules
Voltage Power Modules
Solar Power Modules
Automotive Power Modules
200W Power Amp
IGBT

What Do They All have in Common?
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
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Example Life Expectancies
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
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Semicron Thermal Module
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Failure Mechanisms
o Thermo-mechanical fatigue induced failures
o CTE mismatch
o Temperature swings
o Bond Wire Fatigue
o Shear Stresses between bond pad and wire
o Repeated flexure of the wire
o Lift off (fast temperature cycling effect)
o Heel Cracking
o Die Attach Fatigue
o Solder Fatigue
o Voids
o Device Burn Out
o Automotive- degradation of power
o Solder Fatigue
o Bond wire failure (lift off due to fast temperature cycling)
o Structural Integrity – ceramic substrate to heat sink in thermal cycling
o IGBTs – solder joint fatigue, wirebond liftoff, substrate fracture, conductor
delamination
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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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Example of Substrate Delamination
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
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
Cracks between DBC Substrate and also between silicon die and bond wire
Mitsubishi, “Power Module Reliability”
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Failure Modes - Wire Bond Cracking and Lift Off
o Examples of wire bond
fatigue cracking and
also wire bond lift off
Dynex – AN5945 – IGBT Module Reliability
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Typical Mission Profile
o The stress conditions in the chart are for a railroad
braking application
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IGBT Qualification Tests-Environmental
o Typically,
extensive
qualification
testing is
performed
to ascertain
the
reliability of
the power
module as
shown
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Copper Wire and Temperature Cycling
o Power module industry believes copper wire is more
robust than aluminum
o 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
recrystallization
o Can result in self-healing
o Part of improvement could be more robust fatigue
behavior
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D. Siepe, CIPS 2010
N. Tanabe, Journal de Physique IV, 1995

Aluminum vs. Copper – Temperature Cycling
o Copper clearly superior
100
10
106 107 108 109
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N. Tanabe, Journal de Physique IV, 1995
J. Bielen, EuroSime, 2006

Thermal Aging of Cu Wire Bonds vs. Gold
J. Onuki, M. Koizumi, I. Araki. IEEE Trans. On Comp. Hybrids & Manfg. Tech.
12 (1987) 550
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a. b.
Cu

Points
o Cu is comparable in cost to aluminum but less proven –
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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

Major concerns identified by DfR
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 JEDEC
temp cycling test is sufficient to
drive crack growth
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Die Attach Fatigue
o Failure mechanisms
o CTE mismatch resulting in plastic strain
o Thermo-mechanical fatigue as a result of temperature cycling
o Coarsening
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Typical Thermal Stress Failures in a Die-Substrate Assembly
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Die-Substrate Assembly
Chip, E1,1
Crack at the chip’s surface in its mid-portion
is due to the normal stresses in the chip
Adhesive, E0, 0
Substrate, E2,2
Crack at the chip’s corner is due to the
interfacial stresses
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

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 its
midportion or at its corner located at the interface;
2) cohesive failure of the bonding material (i.e., failure of the die-attach
material); and
3) adhesive failure of the bonding material (i.e., failure at the
adherend/adhesive interface).
An adhesive failure is not expected to occur in a properly fabricated
joint. If such a failure takes place, it usually occurs at a very low load
level, at the product development stage, and should be regarded as a
manufacturing or a quality control failure, rather than a material’s or a
structural 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

E
1 1 1

−

=

2
V
a
1 1 2
t
2
exp
K T T
V
t
B
n
exp(~ 0.063% )
~ 0.51

exp RH
kT
eV
Tf − ×

µ

h
h
L
L
2
( )

×
−
c
s
− ×D × = × + + + +
A G G a
A G
E A
E A
T L F
c c b
s s
9
1 1 2 2
2 1
n
a a

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 OS 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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29

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Concurrent Engineering

Introduction
o The foundation of a reliable
product is a robust design
o 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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Inputs: Parts List
o Color coding of data origin
o Minimizes data entry through intelligent parsing and
embedded package and material databases

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%

ICT Module (optional)
o Uses embedded FEA engine to compute board
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deflection and strain cause by ICT fixture

DFMEA Module (optional
o Uses ODB++ data including net list to create board
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level DFMEA
o Includes customizable spreadsheets for export

o Comprehensive report
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generated in PDF format
o Key summary points
o Detailed inputs and
findings
o User control over contents
o 50-100 page
professionally formatted
document
Automated Report Generation

o Sherlock performs a comprehensive assessment of
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potential wearout mechanisms from a variety of
environments
o 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 prediction
o Allows the user to incorporate traditional empirical prediction
as necessary
Unmatched

Summary - What is Physics of Failure (PoF)?
o Common Definition:
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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 design
o Provides margin
o Mitigates risk from defects
o Satisfies the customer

Automating lifetime simulation of power PCBs

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