Skip to main content

Universal Failsafe IGBT Package for robust power transmission

Final Report Summary - UNIPACK (Universal Failsafe IGBT Package for robust power transmission)

Executive Summary:
Through a mixture of applied research and development, the UniPack project has developed technologies to reduce the manufacturing costs and improve the performance of plastic packaged IGBT (Insulated gate bipolar transistor) modules, in particular those for use in Voltage Source Converters (VSC) for High Voltage Direct Current (HVDC) power transmission. VSC technology enables efficient access to renewable energy sources and economic connection to the electric utilities grid. Trade studies have identified target IGBT module performance parameters for the HVDC market sector and the need to address the influence of gate drive strategies on converter cell/ module performance. Tailoring of IGBT structures for on-state/ switching loss trade-off across each market sector will be demonstrated and applied to develop 4.5KV enhanced DMOS and /or trench IGBT die exhibiting optimum performance for HVDC transmission.
Applied research into a high thermal performance low profile package, designed for low inductance and having fully bonded interconnects has delivered a step improvements in performance / reliability and reduced manufacturing cost. An important characteristic of the package is its capability to fail to short circuit, a feature which is highly beneficial in reducing the system costs.
A working prototype based on the selected concept design has been produced to scale and electrical trials have demonstrated that the device can be used to successfully trigger the short circuit mechanism in ~3.5ms under typical fault conditions (4.5kA 2kV, 1.3ms pulse).

Project Context and Objectives:
The background of UniPack project was to improve reliability in high voltage direct current power transmission (HVDC) containing Insulated Gate Bipolar Transistor (IGBT) modules.
Insulated Gate Bipolar Transistor (IGBT) semiconductor devices are used in the field of power electronics for switching applications, for example in high voltage direct current converters. In the event of a fault, IGBTs are disadvantaged by an intrinsic open circuit failure mode which causes instability of power conversion and frequently results in total loss of power.
Not only does the total power loss (blackout) occur, but failure to open circuit could cause cascade effect in damage of other stacked IGBT modules due to high voltage spikes.
In addition to reducing cascade effects, the increase in remote energy generation (e.g. offshore wind farms) where accessibility is restricted, reliable and highly efficient high-voltage switching is paramount. Therefore, a need exists for IGBTs to incorporate a fail-safe to ensure continuity of power supply and perform efficiently, with the lowest possible losses.
In order to achieve higher blocking voltages than with individual devices, press pack modules have been developed, which are designed to be stacked on top of one another. The pressure applied to each one ensures an appropriate electrical and thermal connection. To protect the delicate chips inside the modules, pistons or springs can be provided so that the pressure on the chips is limited. Any excess pressure is held by the module casing. In the case of a defect or a high current spike the silicon melts forming a conductive channel which generates a stable short circuit. The remaining modules in the stack then take the additional load and a single defective module will not lead to malfunctioning of the entire stack. However, existing failure to short circuit IGBT modules are pressure mounted devices and provide both complex and expensive solutions to the problem of offering a fail to short capability.
UniPack is the development of different device layout using a fail safe switch (FSS) to overcome the intrinsic limitation of IGBT failure. UniPack delivers an IGBT module which has a reliable failure to Short Circuit. The FSS is designed such that both normal and transient but safe fault currents do not initiate the trigger within the switch, but larger fault currents, for example due to semi-conductor failure, will initiate the FSS action and quickly provide a safe short-circuit current path.
Through a mixture of applied research and development, the UniPack project has delivered technologies to reduce the manufacturing costs and improve the performance of plastic packaged IGBT (Insulated gate bipolar transistor) modules, in particular those for use in Voltage Source Converters (VSC) for High Voltage Direct Current (HVDC) power transmission. VSC technology enabled efficient access to renewable energy sources and economic connection to the electric utilities grid. Trade studies have identified target IGBT module performance parameters for the HVDC market sector and the need to address the influence of gate drive strategies on converter cell/ module performance. Tailoring of IGBT structures for on-state/ switching loss trade-off across each market sector has been demonstrated and applied to develop 4.5KV enhanced DMOS and /or trench IGBT die exhibiting optimum performance for HVDC transmission.
Applied research into a high thermal performance low profile package, designed for low inductance and having fully bonded interconnects has delivered step improvements in performance / reliability and reduced manufacturing cost. An important characteristic of the package has been its capability to fail to short circuit, a feature which is highly beneficial in reducing the system costs.
A working prototype based on the selected concept design has been produced to scale and electrical trials have demonstrated that the device can be used to successfully trigger the short circuit mechanism in ~3.5ms under typical fault conditions (4.5kA 2kV, 1.3ms pulse).

Project Results:
The objectives of the UniPack project have been to improve reliability in high voltage direct current power transmission (HVDC) containing Insulated Gate Bipolar Transistor (IGBT) modules.
IGBTs are disadvantaged by an intrinsic open circuit failure mode which causes instability of power conversion and frequently results in total loss of power.
Not only does the total power loss (blackout) occur, but failure to open circuit could cause cascade effect in damage of other stacked IGBT modules due to high voltage spikes.
In addition to reducing cascade effects, the increase in remote energy generation (e.g. offshore wind farms) where accessibility is restricted, reliable and highly efficient high-voltage switching is paramount. Therefore, a need exists for IGBTs to incorporate a fail-safe to ensure continuity of power supply and perform efficiently, with the lowest possible losses.

UniPack has been the development of different device layout to overcome the intrinsic limitation of IGBT failure. UniPack has delivered an IGBT module which has a reliable failure to Short Circuit.

Through a mixture of applied research and development, the UniPack project has delivered technologies to reduce the manufacturing costs and improve the performance of plastic packaged IGBT (Insulated gate bipolar transistor) modules, in particular those for use in Voltage Source Converters (VSC) for High Voltage Direct Current (HVDC) power transmission. VSC technology enabled efficient access to renewable energy sources and economic connection to the electric utilities grid. Trade studies have identified target IGBT module performance parameters for the HVDC market sector and the need to address the influence of gate drive strategies on converter cell/ module performance. Tailoring of IGBT structures for on-state/ switching loss trade-off across each market sector will be demonstrated and applied to develop 4.5KV enhanced DMOS and /or trench IGBT die exhibiting optimum performance for HVDC transmission.
Applied research into a high thermal performance low profile package, designed for low inductance and having fully bonded interconnects has delivered step improvements in performance / reliability and reduced manufacturing cost. An important characteristic of the package has been its capability to fail to short circuit, a feature which is highly beneficial in reducing the system costs.
A working prototype based on the selected concept design has been produced to scale and electrical trials have demonstrated that the device can be used to successfully trigger the short circuit mechanism in ~3.5ms under typical fault conditions (4.5kA 2kV, 1.3ms pulse).

The overall objectives of the WP1 were:
- To simulate thermal, mechanical and electrical operating environment for an IGBT module during normal operation.
- To simulate thermal, mechanical and electrical operating environment for an IGBT module at failure.
- To define the operational envelop for IGBTs for normal operation.
- To define the operational envelop for IGBTs in short circuit mode.
- To develop operating specifications and materials specifications for Transient Liquid Phase (TLP) materials
- To develop operating specifications and materials specifications for the Failsafe switch
In WP1 we defined the target specification for IGBT materials. Task 1 provided Preliminary Specifications for package and component parts, including preliminary IGBT design using existing design rules for Dynex Gate/Die products, construction of IGBT Simulation Model. This has enabled us to evaluate IGBT simulation model against known performance criteria for Dynex IGBT modules in the following tasks. Task 2 has developed thermo-mechanical and electrical models (non-short circuit mode) for IGBT modules and IGBT Press-pack. Three different simulation models were developed: FEM simulation model for determination of thermal image of complete internal IGBT structure, simulation model based on TRAIT method (Thermal resistance analysis by induced transient) for determination of IGBT silicon chip temperature loaded by electric current of different waveforms and SPICE equivalent model for top simulation of presspack IGBT during design developing process. A numbers of simulations were performed using all three simulation models and results compared. Task 1.3 has developed simulation of IGBT module and presswork failure sequence to allow determination of thermo-mechanical condition inside IGBT module during failure (short circuit) operation, developed simulation models from task 1.2 were extended to determine thermo-mechanical stress of some critical parts of IGBT bond wires design (e.g. IGBT emitter bond wires). A numbers of simulations were performed using all developed simulation models and the results compared. Impact resistance requirements for destruction-proof IGBT module package were also established. Task 1.4 has established the phase change parameters for the TLP materials covering selection and analysis of materials that can be used in the solutions proposed in UNIPACK, including TLPs and Ag-sintering nano-materials. Task 1.5 has enabled a specification for the working parameters of the fails safe switch material to be defined.

Significant Achievements in Work Package 1 included
• Comprehensive report covering; Target Dynex Specification including Datasheet, Component Specification Operational Requirements, Short Circuit Features, and Explosive Resistance; Construction of Current Design including Layout, and Sectional Schematic of Constructions.
• 3D Cad model of IGBT module developed. The model forming basis for developing FEM model for performing simulations in different electrical and thermal operation IGBT environment
• Thermo-mechanical and electrical models (non-short circuit mode) for IGBT modules and IGBT Press-pack developed
• Solid structure imaging obtained by FEA and further simulation of transient performed by TRAIT method
• TRAIT simulation model developed giving accurate simulation for any case of current due to power dissipation calculation takes in account all factors related to heat generation in IGBT chip (switching losses, conduction losses, nonlinearity of heat generation [P(t) = UCE (IC; Temp) x IC(t)] ).
• TRAIT method developed allows simulation for any point (or small area with uniform surface temperature) where heat is generated and drained in one axis.
• Simulation model developed provides IGBT producer a tool for IGBT thermal stress analysis in various applications with very different current waveforms beyond standard catalogue data as a base for converters design. This covers an existing "gap" between power semiconductor producer and application designers.
• Simulation is tested on the known thermal image of existing IGBT module and results obtained are demonstrated to be reliable.
• Simulation by means of SPICE software based on Cauer equivalent IGBT thermal model can be used for top level IGBT press-pack simulations. Results obtained compare well with results obtained by TRAIT method relating to collector side cooling
• Simulation of IGBT module and press-pack failure sequence completed
• IGBT fail switch design in bond wire modules can be based on electro mechanical triggered action rather than thermally triggered.
• State of the art of the materials Mat 1 and Mat 2 completed and phase change parameters for the TLP materials and initial specifications required based on the short circuit criteria from Task 1.2 established
• Commercially available nano-Ag sintering materials that form a temporary liquid-like interface at temperature range below 200ºC with minimum pressure have been sourced and analysed and identified for experimental evaluations
• State of the Art in TLPs completed and candidate silicon-interconnect materials that form a temporary liquid-like interface at temperature range of between 190-270ºC that can isothermally diffuse rapidly forming a robust joint:
• A specification for the SMA material has been determined using information from simulation work and standard product test requirements

The overall objectives of the WP2 were:
• To produce fail safe switch material with a conductivity of 3.5x107 Ω-1m-1 (ρ < 28 nΩ m)
• To develop fail safe switch material
• To develop fail safe switch material component with 750A short-circuit operation

In WP2 we understood the characteristics of the fail safe switch material leading to the development of the fail safe switch concepts and integration of these into the IGBT module. More specifically In Task 2.1 we have developed a test-bed to aid in the characterisation of the candidate safe switch material samples including a second test-bench to work under high voltage conditions and assess the behaviour of the prototype safe switch design at and around the failure point of the IGBT. Task 2.2 has involved an extensive literature review and selection and sourcing of commercially available, state-of-the-art safe switch material and used for further investigation. In Task 2.3 we have identified safe switch trigger locations and concept solutions for going forward as well as optimise of the safe switch material. Task 2.4 built on the previous task to incorporate and integrate the safe switch into the IGBT module.
Significant Achievements in Work Package 2 included:
• A Characterisation Test Rig – primary function to characterise a variety of fails safe switch material in regards to time-displacement characteristics and electrical and mechanical properties over a wide range of thermal environments
• VSC test bench – primary function to characterise the fails safe switch in prototype switching device form under high voltage (up to 4.5kV) and current (up to 10kA short duration) fault conditions.
• Literature and experimental based investigations have been combined to enable a choice of materials for further optimisation and development to be determined.
• Identified over 20 potential concept solutions
• Two concept designs have been developed for the fail safe switch in the Unipack application. Both concepts will create a short circuit capable of surviving exposure to fault current for >10ms and normal operating current indefinitely.
• Feasibility studies have been performed on fabricated trigger mechanisms and the results show that both concepts are potential candidates for prototype development
• A working prototype for the fail safe switch has been produced to scale and electrical trials have demonstrated that the fail safe switch can be used to successfully trigger the short circuit mechanism in ~3.5ms under typical fault conditions (4.5kA 2kV, 1.3ms pulse).
• Methods of incorporating the fail safe switch into the IGBT module have been considered and solutions to several potential manufacturing issues have been proposed.

The overall objectives of the WP3 were:
• To develop nano-silver materials that form a joint interface at temperature range of >100°C (Mat #1)
• To develop a creep resistant bond with low temperature processing conditions to resist 10ppm/K CTE mismatches associated with load cycling of IGBT interconnects (Mat #1)
• To develop Transient Liquid Phase (TLP) material to form a joint interface at temperature range of between 190 –270°C (Mat #2)
• To produce a high strength jointing material that can isothermally diffuse within 10 milliseconds to form a creep-resistant joint (Mat #2)
• To optimise electro plating process for the deposition of jointing materials to within ± 1μm
Work in WP3 related to Task 3.1 in identifying potential bonding solutions and acquire materials. This allowed the completion of work in Task 3.2 where we defined process and performance limitations for the first material selected. It allowed commencement of Task 3.3 where we defined process and performance limitation for the second material selected. Work was then extended to measure the thermo-mechanical properties of these two materials as part of Task 3.4 as well as defining the manufacturing process as part of Task 3.5

Significant Achievements in Work Package 3 included:
The analysis of the different mat 1 selected permitted to know the behaviour of these materials at different temperatures, and the range of temperature of the solvent evaporation. The effect of the pressure and temperature related to the silver agglomeration and morphology has also been analyzed. Microstructural characterization has also revealed differences between Ag alternatives
• The design of a jig able to work at different pressure conditions. After several designs, a final jig has been developed able to meet the expected conditions of pressure, and temperature.
• Selection of the Silver coating as the surface condition to perform the joint. DoE accomplished and process parameters influence in the shear resistance analysed. Optimized process parameters selected for future industrialization
• The tests of the joining process at different conditions (temperature, time and pressure) has let us know which is the correct range of them to carry out a proper joint
• The analysis of the metallographic structure of the joint showed us the different phases created, specially the intermetallic ones, and their progression depending on the post treatment they have received
• The effect of the thickness of the intermetallic layer at different conditions has been determined. This allows us to prepare new samples to guarantee not having free Sn at the joint level.
• The thermal shock of the samples at different number of cycles enables us to understand the behaviour of the solder submitted to thermal stresses.
• The metallographic analysis of the materials showed different behaviour between the two materials ; there is a reaction between the silver metallization and the nanosilver when the number of cycles increase
• The porosity of the silver at the interface in both materials could assume the level of stresses produced at the die cracks along the surface during the thermal shocks.
• Analysis of different possibilities for joining fail safe switch contact and the device to the PCB
• Selection of the best configuration for guaranteeing the joint for M1 and M2

The objective of WP4 was to develop a design of IGBT prototype module based on exiting wire type design but with incorporated fail safe switch. To supply components for the prototype and produce a set of modules for testing fail switch functionality and overall IGBT under various testing conditions. Resulting in a successful demonstration of the IGBT module design that fulfills the defined criteria
Significant Achievements in Work Package 4 included:
• The new IGBT module design fulfils the initial defined criteria; the necessary modifications of housing were realised by machining existing piece parts and FSS outer housing dimension and shape were kept unchanged.
• Detailed production drawings were prepared for each prototype part.
• Produced set of the new IGBT module parts to ensure sufficient quantity up to 25 completed modules for tests performed under various conditions.
• Obtained results of testing showing UniPack prototype module characteristics match standard IGBT modules technical data in all tests applied (static and dynamic).
• Tests of integrated short circuit device / fail safe switch (FSS) showed UniPack IGBT prototype module to establish short circuit under failure conditions.
• Produced video material ensuring easy understanding of UniPack short circuit device functioning.
• All video clips are uploaded on YouTube service under public view protected status

Potential Impact:
The UniPack technology will provide an innovative way to avoid power losses in the wide majority of power transmission and distribution applications consisting of a high voltage 4.5kV at 900A IGBT module with an intrinsic failsafe mechanism that will undergo failure to short circuit, thus maintaining continuity of power transmission. The UniPack module will maintain operation for twelve months after failure whilst having a thirty-year life expectancy under normal operation. The UniPack module will have an overall physical footprint of 140mm x 190mm enabling UniPack to be a retro fit product, hence opening up the replacement and maintenance market together with the new build market. The ‘universal’ design of the UniPack device means it will operate in stacked and in non-stacked applications and will ensure continuity of power conversion, preventing cascade events.
Specifically:
- A thermally activated “short circuit” switch to enable a failure to short circuit
- A reliable IGBT interconnect joint to enhance the reliability of an IGBT module
- An IGBT package & busbar circuitry with optimised footprint

UniPack design and technology will have significant impact on the power transmission and power distribution market, particularly: Voltage Source Converters for HVDC applications and Voltage Source Converters for HVAC inverter applications. It will also impact secondary markets for rail traction: Chopper converters for DC traction motor drives, Voltage Source Inverters for Pulse Width Modulation AC motor drives and Pulse Width Modulation rectifiers for AC-DC converters for rail traction
UniPack will provide much needed confidence by ensuring continuity of power supply with the lowest losses possible losses. In doing so, concepts such as “micro-grids”, “interactive networks” and the “internet model” can be realised and highly reliable, accessible and cost effective power supply platforms from smaller, more distributed power sources, e.g. wind farms and alternative energy sources, can be integrated into energy generation and distribution grids.
The technological advances of the UniPack IGBT semiconductor device will give the SME consortium a route to HVDC projects. In this high technology business the market responds well to companies who bring improved facilities, security or reliability to their products, i.e. are seen to be leaders in improving overall quality of power transmission systems. By reducing cost-complexity of power conversion, UniPack will deliver a competitive advantage to the HVDC industry and in-turn a competitive advantage to the UniPack partnership. Therefore, UniPack will enable the European power electronics industry to retain its position in a highly competitive, global market and enable us, the SME supply-chain to deliver a differentiated product that strengthens our competitive position within the semiconductor market
The technical advantage of having a robust failsafe device is estimated to have the potential to increase market share of projects by 10% making our product and our supply chain offering highly appealing to the VSC HVDC and FACTS markets.
The new and innovative module will also be a retro fit unit with no additional cost to the market or users by virtue of its universal packaging. In addition to our primary market, the rail sector is a secondary market which would benefit from the UniPack IGBT devices.
Main Dissemination Activities: The UniPack project website has been active since the end of December 2011 and contains non-confidential information relating to the UniPack project as well as a private area for consortium members. The project website is located at http://www.fp7-unipack.eu/
Trade Shows and Conferences. Members of the UniPack team have attended the following trade shows and conferences where the UniPack project was discussed as appropriate and in some cases the partners have or are planning to present a paper.

PCIM Europe 2013 – the International Conference 14-16 May 2013
The comprehensive conference program included numerous presentations as well as seminars and tutorials, and provided state-of-the-art application know-how on power electronics. Specialists from all over the world introduced their latest products and applications and were available for technical discussions.
ISPSD 2013 - 25th International Symposium on Power Semiconductor Devices & IC's (ISPSD) 26-30 May 2013
The ISPSD brought together power devices and power ICs community experts to enhance and drive forward the research and development of power electronics and its applications. The ISPSD has become the world’s leading conference in the field of power devices and power ICs due to the wealth of technical work presented and they are looking forward to a great conference this year as well.
8th Advanced Technology Workshop on Micropackaging and Thermal Management La Rochelle on 6-7 February 2013
The UniPack consortium attended the 8th Advanced Technology Workshop on Micropackaging and Thermal Management. The event was held in La Rochelle on 6-7 February 2013. This yearly conference has grown year after year by the number of presented papers and attendees
9th Workshop Thermal Management La Rochelle on 5-6 February 2014
Tecnalia Research and Innovation attended and presented a paper at the 9th Workshop Thermal Management in La Rochelle on 5-6 February 2014. Dr Cristina Jimenez from Tecnalia Research and Innovation presented the following paper: Reaction kinetics of Ag3Sn growth in Transient Liquid Phase joining process
ISPSD 2014 - 26th International Symposium on Power Semiconductor Devices & IC's (ISPSD) 15-19 June 2014
The UniPack project will be represented at this conference, which will take place from the 15-19 June 2014 in Waikoloa, USA. ISPSD is the premier forum for technical discussion in all areas of power semiconductor devices and power integrated circuits. Topics of interest include Device Physics, Device Design, Power Devices, Safe-Operating Area, Reliability, ESD, Process Integration, Modelling, Materials, Circuit Design, Power SoC, Packaging, and Thermal Management.
2014 IEEE Applied Power Electronics Conference and Exposition - APEC ~March 16 - 20 2014
The UniPack project plans to attend the APEC conference on March 16 - 20 2014, at the Fort Worth Convention Center, Fort Worth, TX, USA. APEC 2014 continues the long-standing tradition of addressing issues of immediate and long-term interest to the practicing power electronics engineer.
The 7th IET international conference on Power Electronics, Machines and Drives (PEMD) 8-10 April 2014
The UniPack projects plans to attend and present a conference paper at PEMD on 8-10 April 2014 at Midland Hotel, Manchester, UK. The 7th IET international conference on power electronics, machines and drives (PEMD) has a prestigious past and reputation, the event is focused on the latest developments in electrical drives, machines and power electronic systems. The latest research concepts and ideas to technical issues and industrial applications will be discussed and presented.
The 16th Conference on Power Electronics and Applications (EPE’14-ECCE Europe) 26-28 August 2014
The UniPack project will be in attendance and hope to present a conference paper. The EPE’14 ECCE conference ambitiously aims to offer excellent opportunities for networking and building collaborative partnerships among academic and industrial representatives.
Publishable Materials – Brochures, Flyers, Case Study and Exhibition Banner: A range of collateral material have been prepared and produced for the UniPack project which has and will be continued to be used to generate interest in and knowledge of the project within the Power Electronic industry. This collateral has and will continue to be used at a range of trade shows and industry events as well as in targeted mail-shots as appropriate.
PATENT APPLICATION - The detailed IPR strategy developed to ensure that we can effectively exploit project foreground on a pan-European scale has been covered in detail and includes relevant patent applications after a full evaluation of patented competitive technologies and ensuring all novelties are captured

Description of Significant Results Obtained:
• The UniPack Web Portal for partner and public use http://www.fp7-unipack.eu
• An UniPack Project ‘Image’ created which has been used for all project materials
• An ‘Exploitation Path’ has been produced and finalised at the exploitation meeting during the final project meeting.
• A proposed Supply Chain has also been confirmed
• The concept has been demonstrated to partners satisfaction and technical papers are in process of submission
• Suitable patents have been taken out
• Technical papers will be presented at a number of future conferences including 9th WORKSHOP THERMAL MANAGEMENT LA ROCHELLE February 5 and 6 2014

List of Websites:

The address of the web-site is: http://www.fp7-unipack.eu. On accessing the website, the website’s public area pages containing non-confidential information can be viewed.