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IeMRC Flagship Project: Power Electronics 5/22/06 CONFIDENTIAL What is Driving Future Power Electronics? • Power electronics holds the key to annual energy savings of around $400 billion! • Lightweight, high performance products such as mobile computing, home entertainment and power tools • High efficiency, high power density electric drives in products such as air conditioning • Proliferation of automotive and aerospace electronic systems • Increased use of power electronics in transmission and distribution systems • Energy storage systems • Pulsed power 5/22/06 CONFIDENTIAL Common Themes • Increased power densities • Lower electromagnetic emissions • Plug-and-go systems • Extreme operating environments • Higher levels of integration • Lower cost 5/22/06 CONFIDENTIAL Power Semiconductor Devices 100 6kV, 6kA 8kV, 4kA 6kV, 6kA 4kV, 3kA 4.5kV, 4kA 2.5kV, 0.5kA 10 12kV, 1.5kA 4.5kV, 2.1kA Switched Power (MVA) 4.5kV, 3kA 6.5kV 0.9kA 2.5kV, 1.5kA 3.3kV, 1.2kA 1kV, 0.15kA 2.5kV, 0.6kA 1.7kV, 1.2kA 1 ETT 1.2kV, 0.6kA LTT 1kV, 0.3kA GTO 0.4kV, 0.08kA 0.6kV, 0.2kA IGBT 0.1 0.5kV, 0.2kA IEGT 1kV, 25A GCT 0.01 1960 1970 1980 1990 2000 Year 5/22/06 CONFIDENTIAL Power Electronic Packaging • Physical containment for one or more basic component building blocks e.g. semiconductor dies, capacitors, inductors, resistors • Protection from environment e.g. ingress of liquids, dust etc. • Circuit interconnections (internal and external) • Electromagnetic management – EMC issues • Thermal Management Passive components Semiconductor dies Power Module 5/22/06 CONFIDENTIAL Power Electronic Packages Power thyristor 30V, 50A 600V, 30A MOSFET MOSFET Light triggered thyristor 4.5kV, 2.1kA IEGT 600V, 200A Integrated Half-bridge power 1200V, 200A IGBT module module for module aircraft flight surface actuation 5/22/06 CONFIDENTIAL Why Manufacture Power Electronics in UK? • UK based technology and manufacturing capability is currently relatively strong • UK is internationally competitive across the whole supply chain • Many systems are application specific, highly customised and tend to have a relatively high added value • Suited to a technologically advanced manufacturing base and can absorb the relatively high UK labour costs 5/22/06 CONFIDENTIAL Power Electronic Performance Limitations • Semiconductor devices – Silicon max. power device die temperatures from 125°C to 200°C – Silicon Carbide, Gallium Nitride and Diamond > 300°C • Passive devices – Capacitors • Packaging – Thermal cycling – Power density – Environmental 5/22/06 CONFIDENTIAL Anatomy of Typical Package and Heatsink Lead-out interconnect Bond wire Die Encapsulation Solder Housing Direct bonded copper Ceramic Direct bonded copper Solder Copper baseplate Thermal Grease Heatsink Thermal stack has 9 layers, 8 interfaces! 5/22/06 CONFIDENTIAL IeMRC Flagship Project in Power Electronics Aim: To enhance the competitiveness of the UK power electronics industry through improvements to the design and manufacturing capability for high power density systems and in particular those intended for high reliability applications and challenging environments. Programme started 1st July 2005 Duration 42 months Total IeMRC funding £811 k, 5 Academic partners 5/22/06 CONFIDENTIAL Objectives 1. Establish and maintain a roadmap for power electronics modules and associated thermal management systems. 2. Maintain a “technology watch” on emerging technologies for power electronic modules and associated thermal management systems. 3. Develop an enhanced physics of failure approach to the design and qualification of power electronic modules. 4. Establish the feasibility of a range of advanced power electronic module manufacturing technologies and apply selected technologies in a manufacturing environment. 5/22/06 CONFIDENTIAL Academic Partners point analysis tools, power electronics, physics-of-failure reliability module design and predictions, multi-physics failure analysis modelling and numerical optimisation partial discharge high-permittivity heat transfer effects dielectrics and Silicon and thermal Carbide device management fabrication 5/22/06 CONFIDENTIAL Industrial Partners • Dynex Semiconductor • Goodrich • International Rectifier • Morgan Technical Ceramics • QinetiQ • Raytheon Systems • Rohm and Haas • Rolls-Royce • SELEX • Semelab • SR-Drives • TRW Automotive 5/22/06 CONFIDENTIAL Work Packages • WP0 – Management • WP1 – Road mapping • WP2 – Technology watch • WP3 – Reliability and physics of failure • WP4 – Advanced packaging 5/22/06 CONFIDENTIAL Management Structure WP1: Cyril Buttay Project Steering Team 1: Mark Johnson Committee Team 2: Ian Cotton Team 3: David Newcombe Team 4: Mark Johnson Shef. PI WP1 WP2 Team 4 WP4 Team 2 Team 1 Team 3 WP 3 5/22/06 CONFIDENTIAL Work-Package and Team Composition WP1 (Road-Mapping) • Sheffield, Loughborough Team leaders: Cyril Buttay, Paul Palmer WP3 (Reliability) • Team 1 (substrate-level reliability): Sheffield, Greenwich, Dynex, Semelab, Goodrich Team leader: Mark Johnson • Team 2 (partial discharge): Manchester, Greenwich, Dynex, Rolls- Royce Team leader: Ian Cotton • Team 3 (whole module): Sheffield, Greenwich, Manchester, Dynex, Semelab, Goodrich Team leader: David Newcombe WP2/4 (Advanced Packaging) • Team 4 (technology watch/advanced packaging): Sheffield, Greenwich, Oxford, Newcastle, Dynex, Semelab, Goodrich, Rolls- Royce Team leader: Mark Johnson 5/22/06 CONFIDENTIAL WP1 Road Mapping • Two events held: – workshop during November 2005 – on-line follow-up event in January • Data captured in a database using a spreadsheet- based form • Results from the workshop (keywords, issues and metrics) circulated to delegates and discussed at the on-line workshop in January • Consensus that more application focussed events needed • Future event on power electronics for the more electric aircraft is planned for May 2006 5/22/06 CONFIDENTIAL Drivers, Metrics and Keywords Power Electronic System, Power Electronic Converter Top-Down Drivers Topology Lifetime (years, cycles) Cost/kVA Environment (thermal, Customer requirements Power density (kVA/m 3 , vibration radiation) Application requirements kVA/kg) Efficiency Societal demands Standards Hardware Software Legislation Global economics Active Passive Sensor/ control Obsolescence Components Components Cost/unit Cost/unit Cost/unit Functionality Cost/ kVA Cost/ µF, µH etc. Lifetime Integrity Lifetime Lifetime Environment Security Environment Environment Losses GUI On-state loss Losses Accuracy Off-state loss Cost/function Switching loss Thermal performance Bottom -Up Drivers Materials technologies Assembly technologies New concepts Standards Legislation ( RoHS etc.) Metrics Global economics Each metric (e.g. cost/unit) will be Obsolescence influenced by a combination of quantifiable factors: e.g. materials used, assembly technology, device design, thermal management technology etc. 5/22/06 CONFIDENTIAL WP3 Summary of Activities • A series of initial reliability tests were agreed to establish the capability of the current state-of-the-art. • Coupons were acquired for thermal cycling testing of the substrate tiles • Experimental work commenced at Sheffield in October 2005. An interim report has been produced and further tests are ongoing • Greenwich has undertaken a number of thermal cycling simulations to help identify suitable test procedures that will be used to gather failure information for particular mechanisms. • Greenwich has also reviewed the CENELEC and IEC standards on semiconductor power modules 5/22/06 CONFIDENTIAL Summary of Activities • Manchester is working to describe problems relating to partial discharge in power electronic modules • Provide techniques for minimising its likelihood • Provide a physics of failure model • Modelling of modules has been carried out in FEA simulation software 5/22/06 CONFIDENTIAL Thermal Cycling Limitations Repeated heating and cooling of assembly leads to repetitive mechanical stress and eventual failure Flexing of bond wires causes fatigue failure Bond wire (de-bonding) at heel Die Solder CTE mismatch Direct bonded copper causes fatigue failure Ceramic at interfaces Direct bonded copper Solder Copper baseplate 5/22/06 CONFIDENTIAL Substrate Wear-Out • DBC substrates are composed of a ceramic insulator e.g. Al2O3 (aluminum oxide) or AlN (aluminum nitride) onto which pure copper metal is attached • The different expansion coefficients of copper and ceramic lead to mechanical stresses in the ceramic • Cracks originate at the copper/ceramic interface, propagating at an angle of 45 0 • As the crack reaches about one third of the way through the ceramic, the crack direction turns parallel to the substrate surface resulting in conchoidal fracture 5/22/06 CONFIDENTIAL Fatigue failure: Conchoidal fracture %conchoidal fracture vs temperature cycles 100 90 80 70 60 Front of tile % fracture 50 40 30 20 10 Back of tile 0 temperature cycles 5/22/06 CONFIDENTIAL Failure rate of tiles due to temp cycling number of cycles to failure -60 to 150 C -10 to 200 C 1 2 3 4 5 6 7 8 9 substrate tiles 5/22/06 CONFIDENTIAL Ultrasonic Wire Bonding Mechanism 5/22/06 CONFIDENTIAL Failure: Crack at the Bonding Interface Heating process Cooling process Crack induces induces propagation compressive Stress tensile Stress Al Al Substrate (Si) Wire Wire coated with Al Substrate (Si) Substrate (Si) Coefficient of thermal expansion: αAl ≈ 12x αSi 5/22/06 CONFIDENTIAL Wire Bond Degradation 100 cycles -55 to 1500 cycles +125 deg C 3000 cycles • Degradation of bond begins immediately • Crack propagates in the bond wire close to the weld • After 3000 cycles virtually all bond wires have lifted 5/22/06 CONFIDENTIAL Wire Bond Life • Number of cycles to 10000.0 failure can be Thousands of Cycles 1000.0 represented by Coffin- Mason law 100.0 !K 10.0 N 2 ' (T2 $ =% " N 1 % (T1 & " # 1.0 0.1 • K ~ 6.5 10 100 1000 • Rapid degradation in delta T (K) performance with ΔT 5/22/06 CONFIDENTIAL Multi-Physics Modelling at Greenwich Manufacturing Testing Field Failure Mechanisms, Reliability Temperature Stress CFD, FEA, Optimisation… MODELLING TO HELP ESTABLISH DESIGN RULES 5/22/06 CONFIDENTIAL Project Management • Greenwich Team – Chris Bailey IeMRC – Hua Lu POWER ELECTRONICS – Tim Tilford FLAGSHIP PROJECT DTI MODELLING POWER MODULES (MPM) 5/22/06 CONFIDENTIAL IeMRC - Reliability Accelerated Life Testing Interconnect Fatigue, etc 5/22/06 CONFIDENTIAL Solder Interconnect • thermal load profiles Predict effects of: • design parameters: • geometry, material properties 13.5 mm Chip 330 microns Solder 100 microns Cu 0.3 mm Alumina 1 mm Cu 0.3 mm Symmetry 5/22/06 CONFIDENTIAL plane Thermal Cycles • 4 temperature cycles are investigated • Each cycle consists of 15 min ramps and dwells at both low and high temperature extremes. 190 Cycle 1 140 Cycle 2 Cycle 3 T(degree C) 90 Cycle 4 Cycle Tmin Tmax dT 40 1 -55 125 180 2 -25 155 180 -10 3 -40 110 150 4 -10 140 150 -60 0 10 20 30 40 50 60 time(minutes) 5/22/06 CONFIDENTIAL Model Dimension and Materials 6.75mm Thickness 330µm Silicon 100µm Solder 300µm Cu 1mm Alumina 300µm Cu Symmetry plane 5/22/06 CONFIDENTIAL Results: Sn3.8Ag0.7Cu(SAC) crack Plastic work dW distribution at the end of a thermal cycle CYC1 CYC1 CYC3 CYC4 Max(dW)/Mpa 0.4 0.383 0.318 0.296 Nf* 1315.789 1374.193 1655.081 1778.094 *N is actually the life time of an element with a length of 73 microns. f The lifetime of the whole solder joint is much greater. 5/22/06 CONFIDENTIAL Life-time Prediction (SnPb) Number of cycles to crack initiation and crack propagation rate Cycles N dl/dN (µm/cycle) 0 L10000(mm)* Tmin Tmax dT SnAg SnPb SnAg SnPb SnAg SnPb -55 125 180 126 0.135 1.346 -25 155 180 150 0.111 1.105 -40 110 150 162 0.101 1.011 -10 140 150 196 0.081 0.815 * L10000 is the crack length after 10000 cycles. 1. For SnPb, FEA results can be correlated to crack initiation time and crack propagation time 2. For SAC solder this is not available. 3. No lifetime model for SnAg at the moment. 5/22/06 CONFIDENTIAL Residual Stress in Substrate Tiles • Simple models have been used to calculate the stress distribution in tiles • The effects of the following parameters on the stress distribution in a round tile have been investigated: – gap width – radius – ceramic thickness – margin 5/22/06 CONFIDENTIAL Tile with Patterned Copper Symmetry plane Cu AlN 5/22/06 CONFIDENTIAL Symmetry plane Von Mises Stress Distribution Symmetry plane 5/22/06 CONFIDENTIAL Partial Discharge Phenomena • Research student appointed October 2005 • Two day visit to Dynex for student to experience manufacturing techniques / design issues • Literature review completed • FEA Modelling of in-use versus test conditions • Experimental activity commenced – Investigation of acoustic monitoring – Testing of specific materials 5/22/06 CONFIDENTIAL Key Literature Review Findings • For substrates: – Voids within substrates typically dominant failure mechanism – DC breakdown strength of ALN typically double that of DC – Breakdown strengths appear to be non-linear – No significant temperature effect noted • For gels: – Gels also strongly degraded by PD but evidence of self- healing exists – Large influence of humidity on strength of gels – Inverse relationship of temperature with strength of gels 5/22/06 CONFIDENTIAL FEA Modelling • Electrostatic modelling of a module has been carried out for conditions where the module is under test and in-use • When tested in production, all HV terminals bonded to each other and raised in voltage while baseplate is earthed • In use, voltage differences exist between HV terminals • Does the production test actually stress all components of the module? 5/22/06 CONFIDENTIAL Comparison of Test and In-Service E-Fields Substrate in test Substrate in use Gel in test Gel in use 5/22/06 CONFIDENTIAL WP2/4 (Team 4) • A meeting of Team 4 was held on 17th October 2005 to define the future activities of WP2/4. A range of activities were discussed and the following were identified for further action: – Baseplate materials: survey of alternatives – Baseplate-less designs: utilising direct cooling of the substrate tile – Substrate tiles: survey of alternatives – Interconnect: evaluation of alternative techniques such as soldered Cu strip and TLP-bonded Ag foil – Cooling technologies: survey of heat-lane coolers 5/22/06 CONFIDENTIAL Summary of Activities • A survey of base-plate and substrate materials has commenced under the “technology watch” theme • Oxford has performed an extensive review of heat lane literature to furnish understanding of this potentially promising cooling technology – A simple demonstration unit has been designed and is undergoing trials. • Newcastle has initiated work on using deposition techniques for forming substrate-level high-k based decoupling capacitors 5/22/06 CONFIDENTIAL Closed Loop Pulsating Heat Pipes • Constant volume system • Filled with working fluid at saturation (boiling) conditions • Serpentine arrangement of channels or pipes • Hot and Cold side; evaporator and condenser • Boiling of water at the evaporator and bubble formation • Condensing of water at the cold end and bubble collapse • Heat stored in latent heat, transferred by oscillations and condensation • Push and pull of fluid leads to oscillations • Instabilities cause circulations 5/22/06 CONFIDENTIAL Closed Loop Pulsating Heat Pipes • Improved design built and tested • Narrower channels, 2mm => 1.6mm • More loops, 12 => 24 • Non-moving part valves incorporated 5/22/06 CONFIDENTIAL Closed Loop Pulsating Heat Pipes • Non-moving part valves encourage fluid circulation • A number of valve designs were tested using air flow through a stereolith model in ABS plastic. • Teslar non-moving part valve design chosen. 5/22/06 CONFIDENTIAL Closed Loop Pulsating Heat Pipes • Testing • Successful operation in vertical orientation • Difficultly maintaining oscillations when the device is horizontal Pulsating channels Pulsating channels migrate from left to the right Vapour and boiling water droplets Droplets of water feed the evaporator 5/22/06 CONFIDENTIAL Closed Loop Pulsating Heat Pipes • Thermal resistance decreases as heat input is increased • An effective conductivity for the device of 4990W/mK • Twelve times higher than silver Vertical Wet - 100mm PHP Thermal Resistance vs Heat Transferred 1.40 1.20 Thermal Resistance (K/W) 1.00 0.80 y = 28.13x-0.8292 0.60 0.40 0.20 0.00 0.00 50.00 100.00 150.00 200.00 Heat Transferred (Watts) 5/22/06 CONFIDENTIAL Integrated Capacitor Technology • The capacitor is deposited in a series of layers on a DBC (or similar ceramic) substrate. • A multi-layer structure offers higher capacitance (energy storage) per unit volume. High-k dielectric ~500- Metal (e.g. Au/Al) 600 µm ~200- DBC copper layer 300 µm ~500- DBC ceramic layer 600 µm • Area: 60 mm x 20 mm. • Total height: around 600 microns. • Voltage rating: 1000 V • Leakage current less than 1 mA / mm2 at 200°C and 1000 V 5/22/06 CONFIDENTIAL Initial Results for a SiC-based structure • Fabricated on 5x1015cm-3 n-type Cree wafers • Thermally oxidised layer (25nm) • 50nm Ti layer deposited and oxidised at 800oC to give 75 nm TiO2 layer • Palladium gate (50nm thick) deposited • Tested in air in a light-tight box on a hotplate 5/22/06 CONFIDENTIAL Capacitance 250 230 210 Capacitance (pF) 190 170 150 130 1kHz 110 10kHz 90 100kHz 1MHz 70 50 100 200 300 400 500 Temperature (oC) 5/22/06 CONFIDENTIAL Power Electronics Flagship Summary • All academic partners now “up to speed” with staff in place • Technical work packages underway – Road mapping – Reliability and physics of failure – Advanced packaging • Initial progress is promising • Need to maximise gearing through other initiatives e.g. DTI technology programme • Additional industrial partners are welcome to join 5/22/06 CONFIDENTIAL Summary of Current and Potential Future Links Aerospace IeMRC Power EPSRC-funded Innovation and Growth Team: electronics research in SiC AIN, TVP roadmap (ASCENT) DTI-funded Packaging programmes in technology power qualification DTI-funded electronics (5th Advanced research into Call) packaging improved bonding technology (IMPECT) EPSRC-funded research in DTI-funded power module Design for research into technology qualification modelling of power modules (MPM) DTI/RDA funded activities EPPIC (TBA) Foresight Vehicle Faraday 5/22/06 CONFIDENTIAL Web Site, Further Information • http://eeepro.shef.ac.uk/iemrc • Public section has information on the project, its work packages, dissemination of key results etc. • Project partner forum will be used to keep minutes of meetings, project reports etc. 5/22/06 CONFIDENTIAL
"IeMRC Flagship Project Power Electronics"