Reliability of power modules using sherlock

Reliability of Power
Modules Using Sherlock
DfR Webinar
June 16, 2014
Greg Caswell and Craig Hillman
DfR Solutions, LLC
© 2004 -–2200011907000 0 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Agenda
o Introduction
o Power PCB Applications
o Common Issues
o Lifetime Expectations
o Failure Mechanisms
o Virtual Qualification Approach
o Sherlock Solution
© 2004 - 2001907000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Power Modules Are Used in Several Market Segments
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

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

Semicron Thermal Module

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

Bond Wire Fatigue Due to Thermal Effects
Bredtmann, et al, “Options for Electric Power
Steering Modules a Reliability Challenge.”
Automotive Power Electronics, September 2007

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

Failure Modes- Solder and Silicon Cracking
Cracks between DBC Substrate and also between silicon die and bond wire
Mitsubishi, “Power Module Reliability”

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

Typical Mission Profile
o The stress conditions in the chart are for a railroad
braking application

IGBT Qualification Tests-Environmental
o Typically,
extensive
qualification
testing is
performed
to ascertain
the
reliability of
the power
module as
shown

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
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
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

Points
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

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

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

Typical Thermal Stress Failures in a Die-Substrate Assembly
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.

Die Attach Solder Reliability
Marie Curie ECON2 2008

Sherlock
o User Friendly
o Quick
o Flexible
o Intuitive
o Reliable
o One of a Kind
o State of the Art

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

PoF: The Complexity Roadblock

=
E
1 V
1 1
t

h
h
L
L
c
s

−

2
a
1 1 2
2
exp
K T T
V
t
B
n
exp(~ 0.063% )
~ 0.51
exp RH
kT
eV
Tf − ×

µ

2
( )

×
−
− ×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

NPI Cycle Using PoF Modeling

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
29

Concurrent Engineering

o The foundation of a reliable
product is a robust design
o Provides margin
o Mitigates risk
from defects
o Satisfies the
customer
Introduction

o What is Sherlock?
o How will Sherlock help you?
o How is Sherlock unique?
o What can Sherlock do?
o How can Sherlock solve my design challenges?
o How does Sherlock work?
o Conclusion

What is Sherlock?
o Physics of Failure Design Reliability Analysis Tool
o Predicts product failure early in design process, quickly
and accurately
o Electronics-focused –
Used across all
industries

How will Sherlock help me?
o Save time, money, resources
o Produce better, more reliable products
o Reduce warranty costs
o Accelerate product development
o Increase profitability
o Enhance customer satisfaction
Deeper Insight, Earlier in the Design Process

What Can Sherlock Do?
35
IC
Wearout
3D FEA
Thermal Cycling
Shock/Vibe
PTH Fatigue
Solder Fatigue
DFMEA
MTB
ICT
F
Hi-
Fidelity

How Does Sherlock Work?
Phase 1: Data Input
o Parses standard EDA files (schematic, layout, parts list) automatically
o Uses embedded libraries (part, package, materials, solder, laminate)
o Can build box-level finite element analysis model in minutes
Phase 2: Sherlock Analysis
o Produces holistic analysis critical to develop reliable products
o Easy to assign and create standard structures and conditions
o Assessment options include: Thermal Cycling, Mechanical Shock, Natural
Frequency, Harmonic Vibration, Random Vibration, Bending, Integrated Circuit
Wearout, Thermal Derating, Failure Rate, Conductive Anodic Filament, High
Fidelity PCB Model
Phase 3: Report Recommend
o Presents results in multiple formats: Tabular / Histogram / Life curve / Overlay
36

Intuitive
• Easy-to-locate commands
• Industry terminology (parts list,
stackup, pick place, etc.)

Compatible with wide variety of reliability metrics
38

Handles very complex environments
39

Import Standard Design Output Files (Gerber/ODB)
40

Parts List
o Color coding of data origin
o Minimizes data entry through intelligent parsing and embedded package and
material libraries
41

Stackup
o Automatically generates stackup and copper percent (%)
o Library with ~700 laminate materials with 48 different properties
42

Power Module materials Alumina, and Silicon Nitride in database
Die attach reliability is factored in for solder materials
Wire bonds are assessed using a separate calculator
43

Automated Mesh Generation
o Identifies optimum mesh density based on board size
o Expert user no longer required; model time reduced by 90%
44

Features
o Global Part Database
o FEA 3D Model
o Sub-Assembly Analysis
o FEA 3D Viewer
o Result Management
o Component lead modeling
45

46
FEA 3D Modeling
ICT and Shock/Vibration Analysis
• Fully 3D elements for the PCB,
components and mount points
• Increase simulation accuracy
• More reliable meshing algorithm
• Increased analysis flexibility
• Sub-assemblies, heat-sinks,
chassis analysis
FEA Engine
• Multi-core and 64 bit support
• Faster analysis

Sub-Assembly Analysis
o Attach one or more CCA sub-assemblies
to a primary CCA
o Mezzanine cards supported by
standoffs
o Edge-connected cards
o Sherlock automatically
analyzes the main CCA and all
CCA sub-assemblies during a
single ICT or Shock/Vibration
analysis task
o Layer results and component
results automatically
generated for all circuit cards
47
Daughter
Card
Mezzanine Card

48
o High Fidelity PCB
o Newest Feature
o Sherlock can identify and mesh
every copper feature within a PCB
or substrate
o Provides unrivaled insight
into risks due to warpage,
thermal issues, mechanical
loads, etc.

Vibration/Shock/Bending
o Loading
o Mounting
o Direction
49

Thermal Cycling Fatigue
o Cumulative Damage Index (CDI)
o Time to failure
o Thermal profile and Flowtherm results
50

Automated Thermal Derating
o Min / Max temperature assigned to
each part (Operating and Storage)
o Sherlock automatically combines
operating temperatures with thermal
profiles
o Parts that exceed their ratings are
flagged
51

DFMEA
o The only Design Failure Mode Effects Analysis (DFMEA) software
dedicated to electronics
52

Current DFMEA Tools (cont)
o Sherlock – DFMEA module
o Module in Design Reliability Assessment tool
o Imports data from BOM or ODB++ design files (generated by PADS / Expedition)
o Later in design cycle than desired
o Earlier than typically being done today
o Facilitates integrating Reuse into DFMEA
o Highest level of automation – Standardized but customizable
o Common formatting – Can import / export to Excel
o Generates DFMEA by “Subcircuit” (Reuse Function)
o SEV / DET based on Standard Practices (Design Standards, etc.)
o OCC (Occurrence Rating)
o Based on component Reliability Predictions
o Delphi Warranty Database
o Multiple Industry databases
o Future:
o Mission profile will drive reliability predictions / OCC

Failure Rate Class
• Failure Rate
• Failure Modes
• Fail Mode Distribution
Description, properties
Baseline
Component Level DFMEA Generation
Schematic
Parts List
• Circuit Function / SubFunction / Sub-SubFunction
• Reference Designator
• Part Number
Sherlock
Design Assistant
Delphi
Component
Database
(DMS)
Delphi Failure Rate Class
DFMEA
Data Source
Default:
• SEV, OCC, DET
• Failure Cause
• Failure Effects
• Preventions
• Detections
Baseline DFMEA Assumes:
• Typical Application
• Not Safety Related

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

Results Summary
o Results tabulated into a score
o Easy report generation
o PDF Format
o Includes results and inputs
56

o Only software tool that provides a complete life curve
57
PTH Thermal
Cycling Fatigue
Constant Failure Rate
Generic Actuarial MTBF Database
Thermal
Cycling
Solder
Fatigue
Vibration
Fatigue
Over All
Module
Combined
Risk

o Fully Validated
58

Validation – Avionics Manufacturer
o “Following discussions with DfR and confirmation of the PCB
material, from the supplier, I was able to refine the model for the
QFN package and the PCB construction to predict the first failure
of a QFN package at around 700 cycles.
It was interesting to note that the PCB material choice significantly
altered Sherlock’s predicted solder joint life and choice of PCB
material needs to be carefully considered from this perspective.
Subsequent real cycling of the test board has indeed produced a
failure at around 770 temperature cycles and so appearing to add
some (albeit limited) validation of the Sherlock prediction.
With the refined model failures of well soldered BGA joints were
not predicted by Sherlock till around 3000 cycles. Our supplier
has now finished the thermal cycles on the real boards seeing no
BGA failures after about 1200 cycles.”
59

Validation – GPS Manufacturer
o With all material and design information
received, Sherlock predicted failure of
I200 (Actel Microcontroller) in just over
300 thermal cycles of -55C to 125C
60

Why Did I200 Fail Thermal Cycling?
o The Atmel device has two die
o This information is in the Atmel datasheet
o Results in a ball grid array (BGA) with a low coefficient of
thermal expansion (CTE)
o The PCB was constructed with TU-622-5 laminate
o This material has a high coefficient of thermal expansion (CTE)
o This material is in the Sherlock laminate library
o Certain part packages (e.g., BGAs) are very sensitive
to differences in CTE
61

Validation – Satellite Manufacturer
o DfR created an ad-hoc Sherlock project using only a
picture and a component
o No stackup info
o Polyimide
o DfR predicted 1016 cycles to first failure and 1693 cycles
to characteristic life
o Test data showed 750 cycles to first failure
board
o Solder fatigue
calculator was
also used as a
standalone tool

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 design
o Provides margin
o Mitigates risk from defects
o Satisfies the customer

Thank You!
Greg Caswell
Sr. Member of the Technical Staff
DfR Solutions
gcaswell@dfrsolutions.com
© 2004 -–2200011907000 0 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Reliability of power modules using sherlock

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