High-performance low-cost power modules for energy smart network

applications

Partners: Nottingham, Dynex Semiconductor, Alstom Grid

Overview

● Background • Future energy networks • High voltage power electronic converters • Module requirements and state-of-art

● Project overview ● Technical progress ● Conclusions

Energy Smart Networks

CHP

Regional

Waste

Commercial buildings

Industry

Biofuels

Energy storage

Rail

Onshore wind

● By 2050 the UK will be supported by an integrated smart energy

infrastructure2 National/International

Power quality device

Large industry

Energy storage

PV HVDC grid

Offshore wind Conventional

generation

AC grid

H2 EVs

Embedded domestic generation

Smart meters

CHP SMEs Heat

Community

Storage

[2] The UK Low Carbon Transition Plan, HM Government, 15th July 2009.

• Conversion of electricity from one form to another • Control of energy flow to provide for grid quality and security • Underpins the low carbon energy supply chain

Role of Power Electronics

CHP

Waste

Commercial buildings

Industry

Biofuels

Energy storage

Rail

Onshore wind Power quality device

Large industry

Energy storage

PV HVDC grid

Offshore wind Conventional

generation

AC grid

H2 EVs

Embedded domestic generation

Smart meters

CHP SMEs Heat

Storage

Power Electronic Control

Energy Network Power Electronics

● Voltage source converters (VSC) based on IGBT semiconductor switches are the building blocks for existing and emerging network applications e.g. HVDC transmission, interfacing of renewables & energy storage, power quality devices

● In many cases converters must be directly interfaced to ac voltages in excess of 11kV (and up to 400kV)

● Individual semiconductor switches rated to 6.5kV or less so either series connection of devices or series connection of converter cells is needed

+ V

- V

Application Example: VSC-HVDC

Series-Connected IGBTs Conceptually simple circuit Requires PWM High switching losses Harmonic and EMC problems

from PWM

Multi-level circuit Low switching losses Easily “scaleable” Virtually no harmonics More complex controls

+ V

- V

+ V

- V

+ V

- V

+ V

- V = “Chain-Link” Module

U

+½Udc

-½Udc

U

+½Udc

-½Udc

(at least until recently…)

VSC with Series-Connected Cells

+

IGBT2 D2

IGBT1 D1

A

N

P

N

Valve Output Voltage Equivalent to:

● “N+1” redundancy in cells means converter can continue to operate if any one cell fails but…

● Devices must fail to a stable short circuit

Conventional IGBT Packages

● Most IGBTs available as “plastic packaged” power modules: • Multiple solder processes and many ultrasonic bond wires:

assembly is time consuming, labour and energy intensive • Wire bonds and conventional SnAg solder are known

reliability weak points • Large parasitic inductance

● Under extreme overload, each wire acts as a fuse ● High energy disruptive failure leading to an open circuit

VSC Sub-module Components

VSC Sub-Module

IGBT module

IGBT1

IGBT2

R1C1

D1

D2

T1

SW1

Bypass Switch: required to provide stable short circuit in event of module failure

Converter Cell

2m

25cm

IGBT Module Requirements

● High isolation voltage (>10 kV) ● Minimised losses (compromise between on-state

and switching) ● Excellent cooling performance ● High reliability ● Preference for fail-to-short-circuit behaviour in the

event of device destruction

Westcode press-pack

● Pressure contacts between heavy copper pole pieces

● Stable short-circuit failure mode ● Can be “stacked” to form series

strings ● Fully hermetic package ● Expensive solution ● Limited flexibility of layout

ABB press-pack (StakPak)

236x150x26 2.2kg

236x150x26 1.9kg

● Pressure contacts formed by individual springs

● Stable short-circuit failure mode ● Can be “stacked” to form series

strings ● Large number of piece parts ● Expensive

Planar Modules

● Planar or “sandwich” assemblies have potential advantages: • no bond wires • can be cooled from both sides, delivering improved thermal

performance • can be optimised to give exceptionally low parasitic inductance • can offer fail-to-short-circuit operation

● Assembly has historically proved complex and costly, involving a large number of piece parts and assembly processes

C. M. Johnson, et al. “Compact Double-Side Liquid-Impingement-Cooled Integrated Power Electronic Module”, ISPSD '07. pg. 53-56.

Example Modular Assemblies

Research Investigations

● Investigate approaches to a low-cost manufacturing process which delivers application specific benefits of high voltage isolation and a fail-to-short-circuit feature • Target simplified assembly using a reduced number of

bonding processes and piece parts • Retain compatibility (if possible) with existing die

contacting technologies • Take advantage of established PCB technologies for

alignment of small features and reduced part count • Use appropriate high reliability bonding technologies that

will fuse into the semiconductor under fault conditions leaving a robust short circuit

Concept Design

● Options with and without top substrate ● Pressure mount of substrate to cold

plate(s) ● Flex can carry bus-bars/external

connections, passive components

4

1

2

3

5

6

7

8

9

10

1. Lower substrate: 0.3mm Cu on

SiN/AlN 2. Sintered bond 3. Die 4. Upper substrate: 0.3mm Cu on

SiN/AlN 5. Pressure mount housing: Plastic or

metal according to function 6. Sintered via and sintered bond

carrier substrate to upper substrate 7. Carrier substrate: thin (0.1mm or

less) Cu on ceramic or polyimide 8. Cu/other metal interconnect 9. Semi-compliant surround 10. Lead-out 11. Underfill (e.g. epoxy, polyimide)

11

Bonding Technologies

● Conventional SA/SAC solders have limited life due to intermetallic formation and growth

● We have investigated diffusion soldering based on the Ag-Sn transient liquid phase (TLP) process and sintering of Ag nano particles • Stable/no intermetalics • High strength • High remelt temperature • Limited gap filling capability

Silicon

Solder

copper (DBC)

Large IMC with needle-shaped morphology

Bonding Comparison ● Both TLP and sintered bonds have high

strength (shear > 40MPa) ● Both processed at conventional

temperatures (~240-280°C) ● Both require application of pressure

(>2MPa) for void-free bond over a 13mm x 13 mm die

● TLP requires carefully controlled thin interlayers and modified die metallisation

● Sintering can be performed with standard die metallisation

● Range of commercial pastes now available

● Sintering being pursued for onward development

TLP joint: Si die sputtered with ~6µm thick Ag attached on Ag foil plated with ~3 µm thick Sn

Sintered joint: dried at 130°C for 15min, sintered at 240°C and 7MPa for 8 min J.F. Li, P.A. Agyakwa, C.M. Johnson, Kinetics of Ag3Sn growth in Ag-Sn-Ag system

during transient liquid phase soldering process, Acta Materialia, Vol. 58, No.9 (2010) pp. 3429-3443.

Potential Assembly Process

• Flexible PCB/IGBT/substrate joined with sintered Ag joints • Filled vias carry current through polyimide flex and provide

required spatial resolution to contact die features

Conclusions and On-going Work

● Future network power electronics requires robust high voltage IGBT modules

● Planar modules offer the best combination of electrical and thermal performance together with desirable “fail-to-short-circuit” characteristics

● A low-cost alternative to current state-of-art is being investigated

● Sintered Ag nano-particle pastes/film is preferred technology for bonding

● A flexible PCB interconnect provides alignment and circuit functions

● Proof-of-concept coupons produced ● Future work will focus on contact performance under fault

conditions, refinement of assembly process, high voltage isolation and system integration