ieee cpmt webinar power electronics packaging,...

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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

mailto:[email protected]

2 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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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





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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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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





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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


Thermoelectric Element




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Reliable Packaging of Power Electronics





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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




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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.




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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




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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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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




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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




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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.




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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,”




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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




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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




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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




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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.




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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.

ieee cpmt webinar power electronics packaging,...

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