High-performance low-cost powermodules for energy smart network

applications

Partners: Nottingham, Dynex Semiconductor, AlstomGrid

Overview

● Introduction Role of power electronics in future energy networks

High voltage power electronic converters

Module requirements

● Research highlights Sintering of Ag nano particles

High reliability flex interconnect

Fail-to-short-circuit energy improvement

Module assembly and test results

● Conclusions

Role of power electronics

CHP

Waste

Commercialbuildings

Industry

Biofuels

Energystorage

Rail

Onshore windPower qualitydevice

Large industry

Energy storage

PVHVDC grid

OffshorewindConventional

generation

AC grid

H2

EVs

Embedded domesticgeneration

Smartmeters

CHP SMEsHeat

Storage

Power Electronic Control

● Power Electronics underpins the whole low carbon energy supplychain

● Conversion of electricity from one form to another

● Control of energy flow to provide for grid quality and security

Energy network power electronics

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

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

● Individual semiconductor switches rated to 6.5kV or less so eitherseries connection of devices or series connection of converter cells isneeded

+ V

- V

VSC with series-connected cells

+

IGBT2D2

IGBT1D1

A

N

P

N

Valve Output VoltageEquivalent to:

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

● Devices must fail to a stable short circuit

VSC sub-module components

VSC Sub-Module

IGBT module: containing 24 off50A IGBT dies and 12 off 100Adiodes

IGBT1

IGBT2

R1

C1

D1

D2

T1

SW1

Bypass Switch: requiredto provide stable shortcircuit in event of modulefailure

Converter Cell

2m

25cm

7mF (14 kJstored energy)

~2kV

Conventional IGBT packages

● Most IGBTs available as “plastic packaged” power modules:

Multiple solder processes and many ultrasonic bond wires: assembly istime consuming

Wire bonds and conventional SnAg solder are known reliability weakpoints

Large parasitic inductance

● Under extreme overload, each wire acts as a fuse

● High energy disruptive failure leading to an open circuit

3.3kV, 1200A switch (IGBT + diode)

Research overview

● Application 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 devicedestruction (eliminate bypass switch?)

Simplified assembly process

● Research investigations Electro-thermal design (voltage isolation, minimise thermal

resistance & parasitic inductance)

Bonding technologies (improved reliability)

Interconnect technologies (alternatives to wire bonds & bus barsfor improved reliability & low parasitic inductance)

Contact/interconnect technologies (fail-to-short-circuit behaviour)

● Planar structure withcompact volume

● Ability of blocking highvoltages

● Featuring failure toshort circuit behaviour

● No bond-wires and busbars

● Flexible PCBs for achievinginterconnects

● Solid bumps for achievinghigh insulating distance

● Integrated double-sidecooling without baseplate

● Sintering of Ag nanoparticlesas bonding technology

Features

Design of planar power module

Co-simulation of heat transfer andfluid dynamics to obtain thermalresistance

Sintering of Ag nanoparticles

Selection of bonding technology

● Conventional SA/SAC solders have limited lifedue to formation and growth of thermo-mechanical fatigue cracks;

● Sintering of Ag nanoparticles has been selectedbecause it can be carried out on the powerdevices and supporting substrates with thecommon Ni/Ag, Ni/Au or Ni/Pd contactmetallization, and the sintered Ag joints havehigh reliability;

Fatigue cracks formed in Sn-3.5Agsolder joints in a conventional powermodule subjected to thermal cyclingbetween -60 C and 170 C

Investigation in sintering of Agnanoparticles

● Effects of sintering parameters

● Thermo-mechanical reliability of die attachment

● Feasibility and reliability of sintered flexibleinterconnect

Sintering of Ag nanoparticles

Thermo-mechanical reliabilityof die attachment

● Monitoring degradation of the entirebonded area during thermal cycling;

● Sintering parameters leading toshear strength > 40 MPa have beenused to prepare samples:

● Print 100 m thick paste of Agnanoparticles

● Dry the paste at 130 C for 30 min

● Place SiC JFET and diode

● Sinter at 250C and 10 MPa for 5min

● Thermal cycling test has been doneat temperature ranges between -60ºC and 190 ºC;

● Degradation of the die attachmentscharacterised with X-ray CT imaging.

Xradia Versa XRM-500 systemfor the X-ray CT imaging

Test sample

Schematic of sintering process

Sintering of Ag nanoparticles

Thermo-mechanical reliability ofdie attachment● Almost vertical cracks penetrate through

the sintered Ag die attachment – crackedriver bed appearance evidence ofcontinued densification;

● Sintered Ag die attachment degradationsimilar to Pb5Sn solder die attachment.

Good bonding interfaces in the as-sintered Ag joint with9.8% porosity and detected pore size of 0.02 to 0.25 µm.

X-Z plane

X-Y plane

X-ray CT images of the Pb5Sn solder dieattachment

X-ray CT images of the sintered Ag dieattachment

Sintering of Ag nanoparticles

Reliability of flexible interconnect

● Sintered flexible interconnect is much morereliable than the conventional Al wire-bonded samples;

● Microstructual observation from failedsamples indicates that the reliability of thesintered interconnect can be furtherimproved by strengthening the bonding ofthe Ni/Pd finish on the Si diodes.

Schematic geometry and photography of thesample for power cycling test

Power cycling parameters

Juncture temperature: 40°C to 120°C

Typical time per cycle: 5s to 9s

Typical heat time/cycle time: 30% to 50%

Microstructure of the samples failed after power cycling test:(a) 289,211 cycles; (b) 1,682,211 cycles

Failure-to-short behaviour

Alternative interconnect technologies

● Investigate interconnect technologies to increase theenergy level leading to failure to short circuit;

● Increased energy to break the continuity of theconductive paths formed during the test;

● Increase in energy absorbed during failure

Flexible PCB interconnected samples: (a) failedto short circuit; (b) failed to open circuit

The samples of flexible PCB with confined structure:(a) failed to short circuit; (b) failed to open circuit

target

Fabrication of power module

Components to construct one module

2 Si IGBTs and 2 diodeBottom and top AlN-based

DBC substratesBottom and top Flexible

PCBs

Bottom and top side coolersBottom and top side plasticframes

24 +2 +12 metal bumps

Fabrication of power module

Assembly processes

Sinter Si devices and flexiblePCBs on DBC substrates

● Three-step process ofAg sintering and/orsoldering to attach Sidies and bond flexiblePCBs and metal bumpsbetween two DBCsubstrates

● Mounting of thesintered half bridgesample on the plasticframe

● Injection of silicone gelinto the gaps in themounted sample undervacuum

● Installation of doubleside cooler to finalisethe assembly process

Sinter metal bumps on topsides of Si devices

Sinter or solder othermetal bumps and bond

two halves together

Mount the sinteredsample on plastic frames

and inject silicone gel

Install the double sidecoolers

Module test

Forward I-V curves tested fromone IGBT in one sintered sample

● The assembled modules under static electrical testindicate the typical I-V characteristics of IGBTs;

● The assembled modules under overcurrent failurewith an energy of less than 750 J results failure toshort circuit.

Sample failed to short circuit E<750J

Sample failed to open circuit E>750J

Conclusions and further work

● Future network power electronics requires robust high voltage IGBTmodules

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

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

● A flexible PCB provides interconnect and external connections

● Double-side cooled sandwich gives low thermal resistance andmechanical confinement for improved fault tolerance

● Ag-based sintered interconnect offers a route to long-term stable “fail-to-short-circuit” characteristics

● Further work to refine assembly process is ongoing