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Posted on 10 July 2019

Enhancing an IGBT Module for High Temperature & High Current Operations

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 A new generation of modules for demanding applications

The ever increasing requirements on high-power IGBT modules to operate at higher power densities need a series of improvements on the total value chain. During the development phase a team of engineers from different expertises contributes to these improvements. This article explains four of such improvements.

By B. Aydin, C. Corvasce, L. Feller, S. Hartmann, ABB Switzerland Ltd, Semiconductor

 

Introduction

In power electronics IGBTs are gaining more importance since their introduction to the market, not only from the installed number of devices but also from the served applications. Today’s high-power IGBT modules cover a range of applications from industrial inverters up to large traction motor drives, windpower and HVDC converters. The trend in IGBT modules continues to be towards higher power densities. One way to achieve this is by increasing current ratings on the same footprint. This requires higher power dissipations and operating temperatures.

To meet these requirements, each part in the construction of an IGBT-Module has to be optimized. This article explains four of such optimizations made on a HiPak module and proven with a final product rated at 3600A and 1700V.

HiPak Technology

The HiPak modules are high-power IGBTs in industry-standard housings with the popular 190, 70 or 130 x 140 mm footprint as shown in Figure 1. They cover a wide voltage range from 1700 V to 6500 V and a current range from 400 A up to 3600 A. In addition, three different voltage categories for isolation voltages of 4, 6.2 and 10.2 kVRMS are offered. They are built in single IGBT, dual IGBT, dual diode and in chopper configurations.

 

The HiPak IGBT power module family

An IGBT module consists of IGBTs and Diodes, built on substrates that are soldered to a base plate. Terminals are conductor leads, which provide the electrical connection from the electronic circuit on the substrate to contacts outside the module. The chipset and the terminals are protected with a silicone gel moulding, an epoxy layer and the housing.

Implemented Improvements

To achieve reliable operation under higher currents and temperatures, the capabilities of the chipset, terminals, soldering and the silicon gel have been improved. This section explains these improvements in detail.

Chipset

The development of high current modules operating at high temperature is very demanding with regards to both IGBT and diode chip design. Soft and controllable switching behaviour is essential when the chips are utilised in high current modules because the combination of high currents and larger stray inductances will normally result in higher overshoot voltages and snappy behaviour during turn-off. To achieve higher current on the same footprint the 1700V technology platform has been upgraded from SPT to SPT+. Compared to SPT, the SPT+ IGBT technology offers about 15% lower on-state losses while keeping similar turn-off losses as shown in Figure 2.

 

Trade-off curve for SPT and SPT+ for different stray inductances and gate resistors.

The targeted 150°C junction temperature requires stable and reliable operation of the devices well beyond such limit. This imposes a proper optimization of the termination design and of the diode lifetime killing in order to reduce the high temperature leakage. Figure 3 shows the cooler temperature range where both IGBT and diode have been proven to be stable without thermal runaway under the application of a DC voltage of 1400V and 1700V over a time longer than 300sec.

 

1.7kV SPT+ IGBT and diode chip stability test data

Finally, the chip set was qualified by standard reliability tests including HTRB (High Temperature Reverse Bias) performed at full voltage and Tj=150°C, HTGB (High Temperature Gate Bias) and THB (Temperature Humidity Bias 85°C/85% relative humidity).

Package

The packaging technology has to serve four main functions. First, it must provide a current path from the busbar to the chip and then back. Secondly, it has to cool away the heat generated in the module. Thirdly, the package has to isolate the electrical contacts from each other. Finally, the same package needs to ensure its mechanical robustness. Following improvements on the Gel, module soldering and terminals enabled a robust new product with 1700V blocking and 3600A current rating.

High Temperature capable Gel

Silicone gels are used as dielectrics to prevent partial discharge and to seal the system against moisture and atmospheric contaminants. In addition to the trends towards operation at higher junction temperatures, environmental needs are also targeting storage temperatures down to -55°C, where the power modules have to remain fully operational.

The existing insulation material is a silicone gel (gel R) which is specified for an operating range of -40…150°C from the supplier. The new requirements of -55°C … 175°C and the new operational temperature of the chips evoked a verification of the material characteristics of two promising alternatives (gel S and gel E).

The extended temperature ranges of the gels in the datasheet together with the dielectric properties were important requirements for the selection of the potential alternative gels. The potential gel candidates have undergone several tests and investigations.

In order to evaluate the thermal stability of the selected silicone gels a thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out. TGA showed that both the reference gel R and gel S had similar weight losses at 150°C. Gel E has the earliest onset for the weight loss.

DSC analysis as in Figure: 4 showed that Gel S had no phase transition, while gel E had a phase transition peak at -44+- 1°C and the reference gel at -40.6 +-0.6°C. These phase transitions can be attributed to the glass transition temperature of the gel. A harder gel would transmit more thermomechanical stress towards the bond wires and with that to the bond-chip interface.

 

DSC measurements

This has negative influence on the lifetime of the system. In this respect Gel S has no abrupt change in its mechanical properties.

Finally the physical characterization focused on the hardness of the isolation material and on the tackiness or adhesion to other material components of the system. The aim is to have a soft insulating material with a good sealing. The lowest penetration depth is measured for Gel S, which is a negative indication for increased hardness. Comparing adhesion forces between the different gels that of gel E with the other components are clearly the highest followed by gel S and gel R.

To conclude as summarized in Table: 1, gel E was selected to be the best candidate for high temperature operation modules. In addition, the outstanding softness and the tackiness of the gel makes gel E into the most promising alternative to the current gel.

 

Qualitative rating of the investigated gels regarding bulk, thermal and mechanical properties

Module Soldering with Spacer

Higher operation temperatures critically increase the requirements of different packaging technologies in order to maintain high reliability and long lifetime of the IGBT module. Some of the identified lifetime limiting failure mechanisms are terminal solder joints, large area solder joints and wire bond contacts. Therefore an additional step has been introduced to the soldering process of the substrates to the base plate. In this step flat aluminum bonds are soldered on the base plate on positions where substrate edges are attached (Figure 5). These bonds give a reproducible and mechanically sufficient stable spacer to guarantee a minimal thickness of the solder. In this way the tilting of the substrate could be decreased.

 

Base plate with the name of the positions for the spacers

To prove the benefit on reliability, modules with and without spacers have undergone temperature swings. In all modules after cycling cracks in the substrate solder near some substrate corners can be observed. Correlating the crack growth rate with the solder thickness at the corresponding location clearly shows that the locations with the thinnest solder have the highest growth rate. (Figure: 6) Hence the use of spacers improves the power cycling capability.

 

Correlation of crack growth rate and solder thickness

High Current Terminals

With higher current ratings of semiconductor chips, the contribution of resistive losses to the power module’s losses is getting higher. High currents cause unwanted power dissipation through the power terminals to the connected bus bar. Furthermore they can lead to reliability problems due to the overheating of the internal conductor leads. Therefore the current path had to be investigated.

Beside dominant conduction and switching losses, resistive losses occur at several points. On a 2400A 1700V module these losses contribute by 14% to the overall losses. With 39% the terminal contributes the most to the resistive losses. The bond wires, the chip metallization and the wire bonds are minor contributors.

To lower the losses generated in the terminals, the electrical resistance must be lowered. Since there is no affordable material with higher conductivity than copper, the only option left is changing the geometry of the conductor.

The terminals as used in today’s HiPak modules are shown left and the new design on the right in Figure 7.

 

Terminals

As a result a considerable reduction of electrical resistance by 18% is achieved by balancing the current density in the conductor plate and making the current paths shorter. At the same time the mechanical reliability is maintained. With the new terminal design continuous phase currents of up to 1800Arms are reasonably possible.

Electrical Results

To verify the performance of the new 1.7kV SPT+ HiPak module, extensive measurements have been carried out.

Figure 8 shows the IGBT turn-off, turn-on and diode reverse recovery waveforms as measured on module under nominal conditions (900V/3600A) at Tj=150°C. The IGBT and the diode both exhibit controlled switching characteristics as well as short current tails. This behavior is enabled from the combination of SPT buffer design and silicon resistivity used in SPT+ technology, which provides fast switching with low losses and low overshoot.

 

1.7kV SPT+ IGBT module measured under nominal conditions at Tj=150°C

The IGBT turn-off tested at a DC-link voltage of 1200V for a collector current value of 7200A at Tj=150°C is shown in Figure 9 proving the ruggedness of the SPT+ IGBT design when paralleled in the HiPak module.

 

1.7kV SPT+ IGBT module turn-off measured under SOA conditions at Tj=150°C

The short circuit SOA test at Tj=150°C and for a DC-link voltage of 1350V can be seen in Figure10 No thermal runaway after short circuit test has been observed and excellent short circuit capability has been measured at chip level for Tj=150°C and pulse times up to 30us. Moreover the SPT buffer and anode designs employed in the SPT+ IGBT have been optimised in order to obtain a high chip short-circuit SOA capability even at gate voltages exceeding the standard gate drive voltage of 15V.

 

1.7kV SPT+ IGBT module short-circuit characteristics measured under SOA conditions at Tj=150°C

 

 

 

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