IEEE CPMT Webinar Power Electronics Packaging,...
Transcript of IEEE CPMT Webinar Power Electronics Packaging,...
-1-
F. Patrick McCluskey, Ph.D. CALCE/Dept. of Mechanical Engineering
University of Maryland, College Park, MD 20742 [email protected]
(301)-405-0279
Power Electronics Packaging, Reliability, and Thermal Management
IEEE CPMT Webinar
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Innovation Award Winner
A. James Clark School of Engineering University of Maryland Copyright © 2014 CALCE
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Dr. Patrick McCluskey (Ph.D., Materials Science and Engineering, Lehigh University, Bethlehem, PA) is an Associate Professor of Mechanical Engineering at the University of Maryland, College Park. He conducts research at the Center for Advanced Life Cycle Engineering (CALCE) in the areas of thermal management, reliability, and packaging of electronic systems for use in extreme temperature environments and high power applications.
Dr. McCluskey has published more than 100 refereed technical articles on these subjects, and has edited three books. He has also served as technical chairman for multiple international conferences and workshops. He is an associate editor of the IEEE Transactions on Components,Packaging, and Manufacturing Technology.
Dr. McCluskey has provided consulting and short courses for companies in the aerospace, automotive, motor drives, energy exploration and generation, and defense industries. He is a fellow of the International Microelectronics and Packaging Society (IMAPS), and is a member of ASME, IEEE, and SAE.
F. Patrick McCluskey, Ph.D. Dept. of Mechanical Eng./CALCE University of Maryland College Park, MD 20742
Instructor
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Motivation for Thermal Packaging of Power Electronics
IEEE CPMT Webinar Power Electronics Packaging, Reliability, and Thermal Management
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Power Electronics Power electronics are the key to efficient energy
generation, distribution, and utilization. – Vehicles (PHEV, FCV, HEV, EV) – Energy Storage and Power Backup (UPS) – Power Generation
• Efficient Use of Fossil Energy • Renewables: Geothermal/Wind/Solar
– Grid Inverters
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Power Electronics Power electronics refers to systems which process and control the flow of electric energy converting it from one set of voltages, currents, and frequencies to another better suited to user loads.
Power Processor
Controller
Load
Power Input
Power Output
Measurements
Reference
Control Signals
• Energy efficiency 10-15% of generated energy can be saved by widespread use of power electronics.
• Environmentally friendly utilizes non-traditional energy sources (solar/wind)
• Compact and lightweight
Advantages
AC to DC (Computers, Radios) AC to AC (AC motors, Power Systems) DC to AC (Inverters, Electric Cars) DC to DC (Microprocessors)
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Power Converters • Converters/inverters are used
in a wide range of applications. • A converter combines a number
of known power electronic packaging technologies, including: – switching devices
(large semiconductor modules) – control circuitry with advanced
processing – sensors and communication
capabilities – large passive components.
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Increasing Power Densities • Increasing power densities in electronics require
more effective cooling solutions, particularly for power electronics modules. – In modern systems, dissipation levels on the order
of several hundred watts/cm2 are not unusual. • Controlling temperature is critical to device
performance and reliability. – Performance
• Higher power losses →More heat – Slower switching – Higher leakage current – Higher forward voltage
• A positive feedback loop can lead to catastrophic failure. (Thermal run-away)
– Reliability • Many failure mechanisms occur more quickly at higher temperatures. • Others occur more quickly with wide temperature swings.
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IGBT Notional Heat Dissipation
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Dissipation % = Power Loss/Power Output
Blue – at 25°C Red – at 125°C Assuming 50% duty cycle
Heat
Diss
ipat
ion
(%)
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1E+05
1E+06
1E+07
1E+08
1E+09
1E+10
1E+11
1E+12
1E+13
1E+14
1E+15
1E+16
-100 0 100 200 300 400 500Temperature (oC)
Intri
nsic
car
rier c
once
ntra
tion
(/cm
3 )
Ge
Si
GaAs
Intrinsic carrier concentration
Carrier concentration at high temperatures
n kTh
m m ei n p
EkT
G=
−2 2
2
32 3
4 2π ( )* *
• np = ni2
• Higher temperatures are needed to produce the same number of intrinsic carriers in wide bandgap semiconductors
• Intrinsic carrier concentration ni, is the number of carriers generated by thermal excitation across the band gap.
• Intrinsic carrier concentration increases with temperature according to the relationship
Semiconductor Band Gap @ 302K
Germanium 0.67 eV
Silicon 1.11 eV
Gallium Arsenide 1.43 eV
Silicon Carbide 3.26 eV
Gallium Nitride 3.4 eV
Diamond 5.5 eV
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Breakdown Strength
1014 1015 1016 1017 1018 1019
10
100
300
1000
3000
30
3
Bre
akdo
wn
Volta
ge (V
)
Doping Density (cm-3)
Si GaAs
GaP
SiC
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Wide Bandgap Material has Unique Advantages for High Temperature and High Power Devices
• Higher Breakdown Field Higher Blocking Voltage • Higher Saturated Electron Drift Velocity Faster Switching • Higher Thermal Conductivity Better Heat Dissipation • Larger Bandgap Higher Temperature Operation
Silicon 4H-SiC GaN/Si Bulk GaN
Bandgap (eV) 1.1 3.2 3.4 3.4
Dielectric constant 11.9 10.1 9 9
Breakdown elect. field (MV/cm) 0.25 2.2 2.0 3.3
Thermal cond. (W/mK) 150 490 130 230
Sat. Electron velocity (106 cm/s) 10 20 22 30
Electron mobility (cm2/Vs) 1500 1000 1250 1250
Baliga FOM (normalized to Si) 1 223 186 868
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Breakdown Strength
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Leakage Current • SiC MOSFETs show limited leakage up to 200˚C
Intrinsic carrier concentration C. Raynaud, D. Tournier, H. Morel and D. Planson, “Comparison of High Voltage and High Temperature Performances of Wide Bandgap Seminconductors for Vertical Power Devices”, Diamond and Related Materials, vol. 19, no. 1, pp. 1-6, 2010
SiC MOSFET Leakage Current J. D. Scofield , J. N. Merrett , J. Richmond , A. Agarwal and S. Lelsie, “”Performance and Reliability Characteristics of 1200V, 100A, 200˚C Half-Bridge SiC MOSFET-JBS Diode Power Modules, Proceedings of the IMAPS International Conference on High Temperature Electronics, 2010
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SiC MOSFETs can be operated at higher temperatures with lower conduction losses than Si IGBTs
Si vs. SiC Conduction Loss
J. D. Scofield , J. N. Merrett , J. Richmond , A. Agarwal and S. Lelsie, “”Performance and Reliability Characteristics of 1200V, 100A, 200˚C Half-Bridge SiC MOSFET-JBS Diode Power Modules, Proceedings of the IMAPS International Conference on High Temperature Electronics, 2010
Lower On-state Losses
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Summary of Part 1 • Power electronics can generate heat fluxes up to 1 kW/cm2. • Thermal management of these heat fluxes is critical, since un-
controlled temperature increases in power electronics can degrade: – Performance – Efficiency – Reliability
• Wide bandgap devices can be used to widen the temperature range of operation but still require appropriate thermal management.
• Wide bandgap devices require packaging that provides thermal management, and is thermally stable and reliable at high temperatures.
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Thermal Management of Power Electronics
IEEE CPMT Webinar Power Electronics Packaging, Reliability, and Thermal Management
17 Center for Advanced Life Cycle Engineering www.calce.umd.edu
Innovation Award Winner
A. James Clark School of Engineering University of Maryland Copyright © 2014 CALCE
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Thermal Management for Power Electronics
hARcoldplate
1=
IGBT Chip
AlNCu
Cu
Base Plate
Solder
Thermal Grease
Q
Air-cooled Heat Sink or Water-cooled Cold Plate
DB
C
TIGBT
Tbase
Tfluid
fluidbasegreaseDBCsolderdieIGBT ThA
RRRRRQT ++++++= )1(
Approaches to reduce IGBT junction temperature: 1. Use higher thermal conductive solder to reduce Rsolder. 2. Use higher thermal conductive thermal grease to reduce Rgrease. 3. Increase effective heat transfer surface A. 4. Increase effective heat transfer coefficient h (in comparison to conventional
air-cooled heat sink or water-cooled cold plate technologies).
baseR
DBCR
IGBTRsolderR
Example: IGBT (insulated gate bipolar transistor) Packaging
greaseR
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• Thermal greases, pastes and pads are separable interconnects that fill in the gaps between the contact surfaces, reducing thermal resistance. • Commercially available thermal greases, pastes and pads have poor thermal conductivities. (i.e. less than 10 W/mK).
• Bonded interfaces have lower thermal resistance, but also have disadvantages:
• Inability to repair and rework • Thermo-mechanical stress imposed on contact surfaces • Require a solderable heat sink surface
Thermal Interface Materials
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Single Phase Liquid Cooling
Cold Plates • Liquid-cooled cold plates can replace air-
cooled heat sinks • Harness greater cooling capability of
liquids • Cold plates are standard practice in
defense electronics and supercomputers • Require a pump – to replace fan – adding
complexity and may increase size, weight, and cost of system
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Single Phase Microchannel Coolers • Microchannels are fluid passages with
hydraulic diameters ~ 100 µm • Minichannels are fluid passages with
hydraulic diameters ~ 1 mm. • Can be used to form attached “microcooler”
(miniaturized coldplate) or as a part of the substrate
• COTS single-phase water microcoolers provide:
– Heat transfer coefficients 300-1000 W/cm2K
– ∆Tja up to 50K – Flow rates in the 1-2L/min range, and – Pressure drops in the 15-115kPa range.
• Design Issues: – High pressure drop – Larger pump and low COP – Temperature non-uniformity on surface
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Manifold Microchannels
2 stage flow:
• Fluid delivery via large channels – Reduced pressure drop – Directs flow to cooling channels
• Cooling in small, short channels – Better cooling potential – Adjacent to heat source
• Net benefit: – Maximizes cooling potential – Reduces flow resistance – Temperature uniformity – Cooling of 600 W/cm2
measured and higher possible. George M. Harpole and James E Eninger. Micro-channel Heat Exchanger Optimization.
Seventh IEEE Semi-Therm Symposium 1991.
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Motivation for two-phase liquid cooling • Liquid cooling required to meet the
highest cooling needs.
• Two-phase systems, involving evaporation and condensation, are far superior in terms of heat transfer coefficient to single phase systems.
• Evaporative cooling also reduces mass flow rate and reduces temperature variations
• Two phase technologies that are widely used include the following: – Microchannel flow boiling – Spray cooling – Jet impingement
Electronics Cooling 2004
HyperPhysics 2005
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Two-Phase Cold Plate Cooling - Principle • Two-phase cold plate cooling system consists of a cold plate (evaporator), a condenser,
and a mechanical pump. • Heat dissipated from electronic components is transferred to the cold plate, where
some of the liquid vaporizes to carry out a large amount of heat. • The liquid/vapor mixture is pumped to the condenser where the heat is released and
dissipated to the ambient. • The condensed liquid is pumped to the cold plate to complete the closed loop.
Pump
IGBT
Two-Phase Cold Plate
Condenser
Liquid R134a
Two-
Phas
e R
134a
Air-Cooling Fan
IGBT IGBT
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• Toyota Prius motor inverter as the test vehicle; • 12 pairs of IGBT and diode with a total power
dissipation of 2400W on the inverter module; • Power density of IGBT is 120W/cm2 • Power density of diode is 95 W/cm2. • Single-phase cooling using ethylene glycol/water
(50/50) as the baseline and two-phase cooling using R134a is proposed for comparison.
• Cooling targets: (1) maintaining each IGBT temperature as low as possible, and (2) maintaining the temperature of all the IGBTs as uniform as possible.
IGBT: 120W/cm2
Diode: 95W/cm2
Two-Phase Cold Plate Cooling of IGBT
Toyota Prius motor inverter
Aluminum Cold Plate
Copper Base Plate
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Innovation Award Winner
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Single-Phase vs. Two-Phase Cooling Temperature Map on Toyota Motor Inverter
(A) EGW Single-Phase Cooling, PP=0.3W
83.2 87.9 94.5 101.4 108.3 115.3
(C) EGW Single-Phase Cooling, PP=4.0W
75.5 76.0 77.7 79.5 81.3 84.2
68.3 66.6 65.6 65.1 64.5 65.6
(B) R134a-Two-Phase Cooling, PP=0.3W
(D) R134a-Two-Phase Cooling, PP=4.0W
66.6 65.1 64.0 63.2 62.2 63.0
Inle
t In
let
Inle
t In
let
Out
let
Out
let
Out
let
Out
let
(Tinlet = 30°C, Nch = 80, Wch = 0.5mm , Hch = 1.0mm, Ww = 0.5mm)
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• Jet impingement cooling is achieved by passing the coolant fluid through a single nozzle or an array of nozzles directed at the surface to be cooled. The coolant impinges perpendicularly onto the cooled surface at high speed, absorbs heat and reduces the temperature.
Jet Impingement Cooling - Principle
• A very thin boundary layer is formed on the cooled surface under the jet, and very high heat transfer coefficients are achieved in this zone. As fluid starts flowing radially outward, the boundary layer thickens, and heat transfer is adversely affected. This is an inherent disadvantage of using a single jet for cooling.
• Uniform cooling is not possible with a single jet, and hence multiple jets are usually employed for cooling a single chip.
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Jet Impingement Cooling of IGBT • Coolant fluid is in direct contact with the
underside of the DBC substrate. • A jet impingement spray plate containing two
jet arrays is used to provide the cooling; • Each IGBT is 12.7 mm × 12.7 mm at 200W • Working fluid is water with a temperature of
45°C at the jet. • Advantages for jet impingement on the DBC Removal of the baseplate in the package
results in a shorter thermal path and therefore a lower thermal resistance between the power electronics and the coolant fluid;
The overall package can be made smaller and with a reduced weight;
Fewer thermal layers in the package result in fewer interfaces between materials with different coefficients of thermal expansion. Univ. of Nottingham
Multiple-jets Multiple-jets
200W IGBT
AlNCu
DB
C
Solder
Cu
200W IGBTSolder
Spray plate
An example of a sprayplate (right) featuring two 5 ×5 jet impingement arrays intended to cool the two IGBTs on the substrate (left).
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Comparison of Air Cooling, Single-Phase Cold Plate Cooling, and Jet Impingement Cooling
Jet Impingement Cooling of IGBT
Teledyne Scientific, 2007
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• Spray cooling breaks up the liquid into fine droplets that impinge individually on the heated surface, creating a thin liquid film.
• Cooling of the heated surface is achieved through a combination of boiling and thermal conduction through the film and evaporation at the liquid-vapor interface.
• When sprays are used with single phase liquid (without evaporation) droplet impingement induces agitation in the liquid, causing the heat transfer enhancement.
• Multiple sprays are required to cool an array of IC chips or IGBTs for power electronic module.
• Spray cooling offers more uniform cooling on the die but provides lower flux removal than impingement cooling for comparable flow rates.
Spray Cooling - Principle
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• Heat is removed by coolant flow in channels parallel to the cooled area.
• Heat fluxes in the range of 16-840 W/cm2 have been reported.
• Microchannel two-phase cooling solutions have a good thermal resistance and COP, and vary widely in terms of wall heat flux.
• The high COP and low pumping power, combined with the small volume of the fins and channels, makes these solutions advantageous in situations requiring low volume and weight along with high heat transfer capability.
• However, the small size and thermofluid complexity poses challenges in design and operation.
"An Investigation of Flow Boiling regimes in Microchannels of Different Sizes by Means of High-Speed Visualization", Harirchian, T., and Garimella, S. V., Proceedings of Itherm 2008, pp. 197-206
Flow boiling in Micro Channels
High
Low
Heat Flux
Microchannel Flow Boiling
Typical Heat Transfer Coefficient Variations in Two-Phase Flow Through a Tube
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Thermoelectric Cooler Summary Advantages: • Solid state construction, so they are VERY reliable • Can be applied directly where cooling is needed most • Cool objects below the temperature of surrounding ambient • Ability to remove high fluxes with small temperature differences Disadvantages:
• Operating efficiency is very low – it may require 10 W of input electrical power to pump 1 W of heat. • Relatively expensive compared to a fan and heat sink • Heat sinks and fans are still required to dissipate heat from the thermoelectric cooler to the ambient environment. • The thermoelectric itself generates added heat
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Net Cooling Power on TEC:
th
chelecccoolingnet R
TTRIISTq −++−= 2
, 21
Thermoelectric Cooling
Joule Heating
Heat Conduction
Metal
Tc
Current I
Th
Advantages of TEC: High cooling flux Spot cooling ability No moving parts High reliability Compact structure Low weight and small volume
Metal
Metal
Current I
n-ty
pe
p-ty
pe
IGBT Chip
TE Cooling
Joule Heating
TE Heating
Maximum Achievable Cooling:
kSZ
ZTTT chc
ρ2
2max 2
1)(
=
=−
Principle of Thermoelectric Cooler
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Combined Cooling with Thermoelectrics • Objective: By placing a thermoelectric at the center of the back of the
power die it should be possible to smooth out the temperature distribution on the die.
• Advantages: – Small size – Precise temperature control – High reliability – Spot cooling
• Disadvantages: – High cost and low coefficient of performance (COP < 0.3) – Thermoelectric refrigeration loses its advantage for large cooling loads – Potential electric field effects
• Goal: – Isothermalization of die surface . – Reduced maximum on-state die temperature difference over ambient – Reduced maximum die temperature
33 University of Maryland
© 2010
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IGBT Cooling Baseline
AlN Cu
Cu
IGBT Chip
Uniform Heat Flux 100~500 W/cm2
Material/Layer Geometry (mm)
Thermal Conductivity
(W/mK)
Silicon IGBT Chip 10 × 10 × 0.5 120
DBC Top Copper 30 × 30 × 0.32 398
DBC AlN Substrate
30 × 30 × 0.64 170
DBC Bottom Copper
30 × 30 × 0.32 398
Micro-Channel Base
50 × 50 ×6.00 398
High Lead 95Pb5Sn or
Sn4Ag solder 0.10 50
Geometry and Materials Properties
Cooling system: (1) Copper micro channel cooling system, (2) Water as coolant at the inlet temperature of 30oC, and (3) Effective heat transfer coefficient applied on micro channel base ranging from 10,000 to 30,000W/m2-K .
IGBT Package Structure
Liquid Cooled Cold Plate
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Solid/Liquid Hybrid Cooling for IGBT
IGBT Chip
AlNCu
Cu
Uniform Heat Flux100 ~ 200W/cm2
DB
C
Solder
Liquid Cooling Cold Plate
Solder
ΔTMax =35oC
ΔTMax =7oC
0 2 4 6 8 10 12 14120
125
130
135
140
145
150
155
160
165
170h=10000W/m2-K
Tem
pera
ture
(o C)
Diagonal Position on IGBT Chip (mm)
500W/cm2 Heat Flux on IGBT
0 2 4 6 8 10 12 1448
49
50
51
52
53
54
55
56
57
58
100W/cm2 Heat Flux on IGBT
h=10000w/m2-K
Tem
pera
ture
(o C)
Diagonal Position on IGBT Chip (mm)
Thermal Management Target
Position on IGBT Chip
ΔTMax
ΔTMax = 0
ΔTMax ≈ 0Tem
pera
ture
Liquid Cooled Cold Plate
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AlN
IGBT ChipIGBT
AlNCu
Cu
DB
C
Solder
SolderTEC
Liquid Cooling Cold Plate
100W IGBT
0 2 4 6 8 10 12 1445464748495051525354555657585960
TEC(I=6.0A)
TEC(I=5.5A)
TEC(I=5.0A)
TEC(I=4.0A)
Tem
pera
ture
(o C)
Diagonal Position on IGBT (mm)
No TEC
I (A) TAve (°C)
TMax (°C) TMin (°C) ΔTMax (°C)
No TEC N/A 53.4 56.1 49.1 7.0
With TEC
4.0 56.2 58.5 52.8 5.7 5.0 51.3 52.1 50.3 1.7 5.5 49.2 49.9 48.8 1.1 6.0 47.5 48.6 46.9 1.8
Table I: Detailed Thermal Performance (Design #1) (IGBT Power = 100W)
Solid/Liquid Hybrid Cooling for IGBT
Liquid Cooled Cold Plate
Thermoelectric Element
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Reliable Packaging of Power Electronics
IEEE CPMT Webinar Power Electronics Packaging, Reliability, and Thermal Management
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Physics of failure • The basic premise of PoF is that all
failures can be traced to a fundamental degradation mechanism that is operative for the design used and the environment in which it is expected to operate.
• PoF tools model the stress-failure relationship for the dominant environmentally-induced failure mechanisms. These relationships are used to compute expected life and compare it to requirements.
• PoF provides a systematic approach to plan, conduct, implement and evaluate accelerated life tests, especially under multiple load conditions.
Implementing Physics of Failure • Define realistic product requirements. • Define the design usage profile. • Characterize the design. • Conduct a virtual qualification • Identify potential failure sites and mechanisms. • Determine overstress and destruct limits • Develop an accelerated test plan • Characterize individual failure mechanisms • Conduct an accelerated life test and update
failure models • Conduct a reliability assessment
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Measuring Reliability • The preferred metric is Failure Free
Operating Period. • Failure free operating period
(FFOP) of a system is defined as: “a period of time (or appropriate
units) during which the system, operating within specific environmental conditions, is functional without encountering failures.”
• There are many distributions that can be used to represent the failures. Exponential, normal, log normal, gamma, Weibull etc. are examples of such distributions. Failure free operating period is a period of time when the probability density function is zero.
Probability Density Function for FFOP
Distributions with FFOP
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Failure distributions for individual failure mechanisms
FFOP/MFOP can be estimated from knowledge of the dominant failure mechanisms using PoF simulation/testing, to desired confidence levels
MFOP/FFOP
Time to Failure (durability)
MTTF
Increasing Stress
63.2% of products will have failed for exponential distribution and 50% for normal distribution
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Comprehensive System PoF Reliability Assessment
Overall system
Part1 Partn
Estim
atio
n of
the
over
all s
yste
m
Failure mechanism1
Part2 Parts arranged in different configurations e.g., series, parallel
Failure mechanism2
Failure mechanism2
Failure mechanism2
Failure mechanismn
Failure mechanismn
Failure mechanismn
Failure mechanism1
( )ii
fi
ii
f
ii
f rd)r(d)R(ψρ
ψ−ψ=
ψρψρ−
≈ψρ
ψρ−=ε
Sub-system1 Sub-system2 Sub-systemn
mKAdNda
∆⋅= Nf = 0.5 (∆γ/2εf)c
PoF mechanism
identification
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• Dice interconnected with 125-375 µm diameter wire.
• Dice soldered to a thick metalized ceramic substrate (e.g. DBA, DBC) which is soldered to a heat spreader.
• Most heat (>85%) dissipated from the back of the die through the substrate to the heat spreader.
• Heat is then transferred from the heat spreader to the heat sink through a thermal interface material.
Substrate Wirebond Ceramic Die
Key Features IGBT module
Packaging Strategies “Traditional” Wirebonded Module
43 Center for Advanced Life Cycle Engineering www.calce.umd.edu
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Identify life cycle profile • A life cycle profile is a forecast of events
and associated environmental and usage conditions a product will experience from manufacture to end of life.
• The phases in a product life cycle includes manufacturing/assembly, test, rework, storage, transportation and handling, operation, repair and maintenance.
• The description of life cycle profile needs to include the occurrences and duration of these conditions.
• Life cycle loads include conditions such as temperature, humidity, pressure, vibration, shock, chemical environments, radiation, contaminants, current, voltage, power and the rates of change of these conditions.
44 Center for Advanced Life Cycle Engineering www.calce.umd.edu
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A. James Clark School of Engineering University of Maryland Copyright © 2014 CALCE
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Load Transformation
Relay strain gage PSD @ 25c
Frequency (Hz)
0
2
3
4
5
6
30g20g
30g
20g
Based on the response of the structure to applied and internally generated loading conditions, stresses for use in the failure models are calculated.
Curvatures and Displacements Mode Shape and Frequencies
Housing Fixture
Global Simulation Model
Local Simulation Model
Anticipated Loads
Temperatures
45 Center for Advanced Life Cycle Engineering www.calce.umd.edu
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A. James Clark School of Engineering University of Maryland Copyright © 2014 CALCE
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Failure Models
),...,( nif xxFt =
Failure Model
Cycles to Failure (Log Scale)
Stra
in
(Log
Sca
le)
Elastic Plastic
Elastic and Plastic
Fatigue curve (shown above) is one form of failure model that may be applicable to electronic hardware.
Models which describe failure process at the material level are called physics-of-failure models. Models which are based on curve-fitting of product level test data are called empirical models. Failure models have the form
where are the parameters obtained from design capture, life cycle load characterization, and load transformation (stress analysis).
ix
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Innovation Award Winner
A. James Clark School of Engineering University of Maryland Copyright © 2014 CALCE
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Temperature Related Packaging Failures in “Traditional” Power Modules
• Wirebonds – primary failure site for high frequency power cycling. – Wire flexure fatigue – Wire lift-off
• Die attach –primary failure site for narrow temperature range passive thermal cycling – Attach fracture and fatigue
• Substrate – primary failure site for wide temperature range passive thermal cycling – Copper delamination – Substrate fracture and fatigue
[1] K. Meyyappan. P. McCluskey, and P. Hansen, Adv. Elect. Pkg., Proc. Interpack 2003. Paper # IPACK2003-35136 [2] Dasgupta, A., Oyan, C., Barker, D., and Pecht, M., ASME J Elect Pkg, vol. 114, no. 2, 1992. pp. 152-160.
[1]
[2]
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)r( fi κ−κ=ε
where κi and κf are the curvatures of the wire before and after heating.
( )ii
fi
ii
f rdRψρ
ψψψρ
ψρε
−≈
−=
)( δs = ρiψi ≈ ρfψf
Wire Flexural Stress Model • Wires subjected to thermal cycling will undergo flexure due to thermal expansion mismatch between the wire, chip, and substrate. • For wedge bonded wires, failure will occur at the heel of the wire on the higher pad. • For ball bonded wires, failure will occur at the heel of the wedge bond or the neck of the ball bond. • Using the theory of curved beams and considering a small section of the wire before and after heating, strains can be expressed in terms of curvature as1:
1K. Meyyappan, P. McCluskey, and P. Hansen, "Wire Flexural Fatigue Model for Asymmetric Bond Height," Proceedings of the 2003 InterPACK Conference, held in Maui, HI, July 7-11, 2003. Paper # Interpack2003-35136
48 Center for Advanced Life Cycle Engineering www.calce.umd.edu
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A. James Clark School of Engineering University of Maryland Copyright © 2014 CALCE
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Copper wire bonding • Ultrasonic wedge bonding • Good quality bond if made to ENIG • Poor quality bond if made to aluminum
bond pad which is too soft – leading to: • Crushing of the bond pad • Cracking/cratering of the silicon
• Able to carry higher levels of power due to 40% lower electrical resistivity and higher melting temperature
• Better heat transfer (higher k) • Better CTE match to Si than Al has • Less susceptible to annealing at high
temperatures (higher melting point) • Improved Reliability
Siepe, D., Bayerer, R., Roth, R., “The Future of Wire Bonding is? Wire Bonding!,” CIPS 2010
49 Center for Advanced Life Cycle Engineering www.calce.umd.edu
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A. James Clark School of Engineering University of Maryland Copyright © 2014 CALCE
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Concerns with Copper Wirebonding and Green Molding Compounds
• Corrosion – The native Al2O3 on the Al bondpad surface is broken
by ultrasonic bonding – CuAl2 begins to form where the Al2O3 is broken – Cu9Al4 then forms as a second intermetallic compound
(IMC) and grows simultaneously with the CuAl2
– Moisture and Cl- from the encapsulant attack IMCs. • Oxygen penetrates from the edge of the ball and oxidizes
CuAl IMCs • CuO is a resistive layer that reacts to induce an open. • Hydrolysis of CuAl IMC and AlCl3 forms resistive
Al2O3
• Outgassing of H2 during the hydrolysis of CuAl2 induces cracking at the Cu ball and pad interface starting at the bond periphery and moving to the center of the ball.
50 Center for Advanced Life Cycle Engineering www.calce.umd.edu
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A. James Clark School of Engineering University of Maryland Copyright © 2014 CALCE
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Concerns with Copper Wirebonding and Green Molding Compounds
• Thermomechanical Effect – Component Level (Wirebonds):
• Thermal mismatch between Cu wire (16.5 X10-6/K) and green encapsulant (7 X 10-6/K) causes high stresses in Cu wire bonds during temperature cycling tests
• The stresses are not present for gold wire (14.2 X 10-6/K) in conventional encapsulant (13 X 10-6/K).
• Stresses are exacerbated by the higher stiffness of both the wire and the green molding compound
• Repeated deformation per cycle results in fatigue cracking of the Cu wire bond at the neck of the bond.
– Assembly Level (Solder Joints) • The lower CTE and higher modulus of green molding
compound also creates greater thermomechanical stress on solder joints to organic printed wiring boards reducing life by as much as 20%.
B. Vandevelde, et.al., “Green Mould Compounds: Impact on Second Level Interconnect Reliability,” 2011 EPTC
B.Vandevelde, “Early fatigue failures in Copper wire bonds inside packages with low CTE Green Mold Compounds,”
51 Center for Advanced Life Cycle Engineering www.calce.umd.edu
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Alternatives to Wirebonding Large Area Solder/Sintered Interconnect
• Chip bonded top and bottom with a permanent attach. • Eliminates wirebond failures • Supports double sided cooling • Need high temperature attach
robust against delamination and cracking
Semikron SKiN Module
Beckedahl, P., “Power Electronics Packaging Revolution Without Bond Wires, Solder, and Thermal Paste,” Power Electronics Europe, No. 5, July/August 2011.
Industrial Standard
SKiN Target
Single Side Solder
Skin Module EOL Skin Module Ongoing Test Benchmark EOL
52 Center for Advanced Life Cycle Engineering www.calce.umd.edu
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Die Attach Fatigue • Failure site: Die attach • Failure mode: Loss of adhesion between the die and attach leading to open circuit, thermal runaway, mechanical failure. • Failure mechanism: Voids and microcracks initiate at the edge of the die and propagate through the attach layer during temperature cycling due to shear and tensile stresses caused by thermal expansion mismatch between the die and substrate.
• High cycle fatigue (uncracked) Stresses below the yield strength
• Low cycle fatigue (uncracked)
Stresses above the yield strength
Basquin’s law
Coffin-Manson and others
( )βσ∆= CN f
Nf=0.5(∆γ/2εf)1/c
53 Center for Advanced Life Cycle Engineering www.calce.umd.edu
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Device Reflow Attachments Solder Tm Issues
Sn37Pb 183˚C - Low Tm, Regulatory Restrictions SAC305 217˚C - Low Tm, Short Fatigue Life Pb5.0Sn2.5Ag and other high Pb
296˚C - High process temperature - Regulatory restrictions
Bi-Ag Alloys 262˚C - Small elongation, brittle - Limited wetting capabilities - Low thermal conductivity
Au20Sn 280˚C - High process temperature - High cost Au12Ge 361˚C
Au3.2Si 363˚C Zn6Al 381˚C - Complicated processing
- Limited wetting capabilities - High process temperature
Zn5.8Ge 390˚C
54 Center for Advanced Life Cycle Engineering www.calce.umd.edu
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A. James Clark School of Engineering University of Maryland Copyright © 2014 CALCE
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Sintered Silver Powder Principle: Use of sintering to create a high melting temperature
lead-free, solid metal joint at a lower processing temperature. Approach # 1: Combine moderate range processing temperatures (225°C - 275°C)
with 30-40 MPa pressure to convert a silver powder paste into a porous solid joint. Order of magnitude better power cycling reliability than soldering [1].
Approach # 2: Sinter over a similar temperature range (225°C - 275°C), but with a
reduced pressure (3 – 5 MPa) via the use of nanoparticles. The driving force replacing pressure is a reduction of surface energy. Reliability is a function of particle concentration in the paste and the bondline thickness but can be an order of magnitude greater than soldering [2].
Sintering chamber
Typical carrier
[2] Lu, G.Q., Zhang, Z., “Pressure Assisted Low-Temperature Sintering of Silver Paste as an Alternative Die Attach Solution to Reflow,” IEEE Trans. on Electronic Packaging Manufacturing, 25 (2002), No. 4
[1] Schwarzbauer, H., Kuhnert, R., “Novel Large Area Joining Technique for Improved Power Device Performance,” IEEE Transactions on Industry Applications, 27 (1991) No. 1. pp. 93-95.
55 Center for Advanced Life Cycle Engineering www.calce.umd.edu
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Introduction to TLPS TLPS (Transient Liquid Phase Sintering) is a liquid-assisted sintering process
during which a low melting temperature constituent, A, melts, surrounds, and diffuses in a high melting temperature constituent B.
Intermetallic compounds with high melting temperatures are formed by liquid-solid diffusion
TLPS systems can be processed at low temperatures but are capable of operating at the high melting temperatures of the intermetallic compounds.
A B A
B A B
B
B B A A+B
Initial Arrangement Heating to Tp Isothermal Hold Final Alloying
B
A
B
B
A
B
A+B
Paste-based:
Layer-based:
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Layer Transient Liquid Phase Sintering • Layers of tin and copper are stacked. Tin melts and diffuses into
copper creating a Cu-Sn intermetallic joint.
• Greater than an order of magnitude increase in reliability over soldering.
Guth, K., Siepe, D., Gorlich, J., Torwesten, H., Roth, R., Hille, F., Umbach, F., “New Assembly and Interconnects beyond Sintering Methods,” PCIM 2010.
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Paste-based vs. Layer-based TLPS
Paste-based Layer-based Kang et al., J. Electron. Mater. 31, 1238 (2002)
• Several layer-based TLPS systems with Sn or In as the low-melting point constituent have been demonstrated: (Ag, Au, Cu, Ni)-Sn, (Ag, Au)-In
• One major disadvantage of the layer-based approach is the limited joint thickness due to Fick’s second law of diffusion: lIMC ~ t½
• The paste-based approach overcomes this limitation by simultaneous sintering throughout the entire joint. Residual high melting temperature metal can be present within joint.
• Joint densification by capillary forces • But, inadequate densification may
lead to voiding
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TLPS Attach Technology
Ni-Sn Joint (Tm:< 30min, P < 1MPa)
Softening point - shear strength <10MPa
0
2
4
6
8
10
12
14
16
Cu60Sn Cu50Sn Cu40Sn
Shea
r St
reng
th in
MPa
25˚C 400˚C 600˚C
• Increasing Cu-content in Cu-Sn joints produces joints that retain their shear strength to T > 400°C, but reduces strength at room temperature.
• Cu-Sn joints can be made in less than 10 minutes at P < 1 MPa, and T < 300°C.
• Ni-Sn joints made in less than 30 minutes at P < 1 MPa, and T < 300°C retain shear strength to T > 600°C.
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Direct Bond Copper
Crack path visible in DBC Areal Extent of Crack Shown in C-SAM
DBC cracking is observed to initiate at the copper-ceramic interface under large ∆T thermal cycling, due to local CTE mismatch. Conchoidal cracks then propagate through the ceramic. Suggested Solutions: • Dimples • Ceramic Strengthening Additives • Thinner/Graded Metallization AlN-DBC cracking after 50 cycles of -55°C - 250°C
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• Conductive Filament Formation – Creation of thin metal conducting
filaments between traces and vias on the board at high voltage when subjected to thermal cycling and humidity
• Solder Fatigue – PTH and SMT components
• PTH/Via Fatigue – Fatigue cracking of the walls of a plated
through hole or via as a result of thermal cycling. Crack can propagate around the circumference of the plated through hole (PTH) or Via when cyclic stresses exceed the fatigue strength of the copper wall
• Corrosion • Creep Corrosion/Dendrite Growth
– Electrochemical metal degradation
Power Board Failure Mechanisms
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Modeling SAC Fatigue at Higher Temperatures
x 100x 10x 1.00
5.00
10.00
50.00
90.00
99.00 Probability - Weibull
Time, (t)
Unr
elia
bilit
y, F
(t)
8/24/2004 09:12 CALCE Center
Weibull Lccc68 Sn/Ag W2 RRX - SRM MED F=16 / S=0 Lccc68 Sn/Ag/Cu W2 RRX - SRM MED F=16 / S=0 Lccc68 Sn/Pb W2 RRX - SRM MED F=15 / S=0 Lccc84 Sn/Ag W2 RRX - SRM MED F=16 / S=0 Lccc84 Sn/Ag/Cu W2 RRX - SRM MED F=15 / S=0 Lccc84 Sn/Pb W2 RRX - SRM MED F=16 / S=0
SnPb
Pb-free
Tmax = 125°C, ∆T=100°C George, E., Das, D., Osterman, M., and Pecht, M., “Thermal Cycling Reliability of Lead Free Solders (SAC305 and Sn3.5Ag) for High Temperature Applications,” IEEE Trans. Device and Material Reliability, Vol. 11, No. 2 (2011). Pp. 328-338.
• SnAgCu is predicted to exhibit shorter lifetime for conditions of -15°C to 150°C than for conditions of -40°C to 125°C, revealing that SAC solder performs more poorly at higher mean temperatures.
• Cu is added to SnAg in SAC to pin the grain boundaries and reduce creep, but creep resistance can still be poor in SAC alloys.
• Sn37Pb outlasts SAC 305 when thermally cycled over a temperature range from -40°C to temperatures in excess of 150°C.
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Example: Electro-Thermal Simulation • Electrical and thermal templates for each component are developed and interconnected.
•Electrical •Si IGBTs •Si MOSFETs •Si PiN Diodes
•Thermal •Die •Die Attach •DBC Layers •Baseplate •Ambient
• The power dissipation in the electrical components will provide the heat for the thermal template input nodes. • Thermal models are validated using NIST high speed transient thermal imaging (TTI) and high speed temperature sensitive parameter (TSP) measurement technologies.
TJ
TH
TC
TA TA
TC
TH
TJ
TJTJ
TH
TC
TA TA
TC
TH
G1
G2
S1
S2D1
D2
ElectricalModel
ThermalModel
62
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Electrothermal -Mechanical Modeling • Electrothermal Model Created
• Power Loss Temperature Increase
• Degradation Model Added • Temperature Increase Attach Fatigue • Fatigue Increased Thermal Resistance • Increased Resistance Temp. Increase
• Precursor Parameter Identified • Temp. Increase Voltage Increase
• Damage Correlated with C-SAM • Monitor Voltage Increase • Estimate Remaining Life
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Conclusions • Power electronics is the critical enabling
technology at the intersection of renewable power generation, reliable power distribution and transmission, and efficient power utilization and storage.
• Issues of compact and high power density packaging, thermal management and reliability are the most important research areas for realizing the full potential of power electronics.