05. Februar 2020, 08:00 Uhr | Ralf Higgelke
Wide-bandgap semiconductors and electromobility pose completely new challenges for packaging and interconnection technology. For this reason, the European Center for Power Electronics (ECPE) organized a workshop on "Advanced Power Packaging – Power Modules 2.0". DESIGN&ELEKTRONIK was on site.
For two days, over one hundred participants discussed in Hamburg the latest innovations in power modules. This is appropriate, because the emergence of silicon carbide and gallium nitride – the so-called wide-bandgap semiconductors – and the shift of more and more car companies away from internal combustion engines and towards electric drives require new approaches. The program was therefore divided into several categories:
In his keynote address, Prof. Eckart Hoene introduced the audience to these challenges and presented some of the solutions developed by his team at Fraunhofer IZM over the years. Previous module designs, which were developed in view of the relatively slow-switching silicon IGBTs, offer good thermal connection, but the electrical interface must be significantly improved, the speaker emphasized. To reduce the high overvoltages caused by the commutation current via the comparatively large external DC link inductance (approx. 10 nH), Hoene proposes to close the commutation loop by ceramic capacitors (26 nF) integrated into the module. At the moment of switching on, these relieve the large external DC link capacitor so that the high di/dt does not flow through the DC link inductance. When being switched off, these capacitors absorb the energy from the DC link inductance, which again reduces the overvoltage. Hoene put his finger in the wound on one thing: The inductance of a module, as it is usually measured or specified by manufacturers today, gives far too optimistic values.
System Integration and Die Attach
Prof. Ulf Schümann from the Kiel University of Applied Sciences discussed the challenges for power electronics, if it has to be integrated into a automotive drive train. He presented the InMove project, in which the university, together with Volkswagen and other partners, developed an 80 kW inverter for an electric vehicle. The inverter was to be attached to the front of the electric motor to allow the entire structure to fit into a cylinder with a length of 34 cm and a diameter of 11 cm.
The researchers used 1700 V modules from Danfoss Silicon Power with their proprietary ShowerPower-3D liquid cooling system and a particularly flat, hermetically sealed electrolytic capacitor from FTCAP in the DC link. A particular challenge was to integrate all these components mechanically and provide the electrical connections in such a way that the form factor was met.
Figure 1: Structure of a standard module (left) and the IsoPower module from Danfoss Silicon Power (right) .
Dr. Aylin Bicakci from Danfoss Silicon Power dedicated her presentation to the thermal interface of power modules. In order to further increase power density, the thermal resistance has to be further reduced. This means, however, that the ceramic substrates commonly used today have to become thinner, but this is not possible for mechanical reasons. Dr. Bicakci presented a solution called IsoPower, which is based on two thick copper layers that are electrically insulated from each other just by a thin film of organic material (Fig. 1; ). The power semiconductors are sintered onto the upper copper layer. This reduces the chip temperature of such an IsoPower module by 25 K compared to a standard module with ceramic substrate.
Tilo Welker explained a new module developed by Rogers and Prof. Hoene's team at Fraunhofer IZM. To keep the leakage inductance as low as possible, the team uses a multilayer ceramic substrate with vias. Thick copper layers ensure that heat losses are spread and quickly dissipated. Thanks to the PSiP concept (Power System in Package) with DC link capacitors integrated in the module, the module (850 V/100 A) achieves a di/dt of 12.6 kA/µs when switched on and a du/dt of 51 kV/µs when switched off. At the same time, the voltage only overshoots by less than 20 V, which indicates a stray inductance in the commutation loop of only 1.6 nH (L = ∆U/(du/dt)).
Dr. Jacek Rudzki from Danfoss Silicon Power took the participants on a time trip through 35 years of sintering technology for power semiconductors. He began with Herbert Schwarzbauer, who first sintered a disk thyristor onto a molybdenum plate using silver powder at Siemens in 1987. Sintering chips onto a DCB substrate (Direct Copper Bond) and the substrate in turn onto a baseplate, driven, among other things, by Semikron, began to appear just in the second half of the 2000s. The latest developments are direct sintering on aluminium substrates, laminating wafers with a sintering paste and sintering with copper. Contacting the transistor connections on the top of the chip using sintering technology (top contacts) is now also feasible.
Figure 2: The DTF (Die Transfer Film) process, where a layer of sintering paste is laminated onto the wafer before it is separated .
Michiel de Monchy from Alpha Assembly dug a little deeper into the sintering process. He used diagrams to show how contact pressure and the grain size of the sintering powder affect the strength of the joint. Generally speaking, the process time decreases the higher the contact pressure and the finer the grains are. The speaker illustrated how much this connection technology increases reliability by subjecting two power switches in the TO-247 package to a load cycle test (power cycling). While the conventional component with soldered and wire-bonded connections reached its end of life after only 12,000 cycles at a rated current of 130 A and a temperature swing of 85 K, the double-sided sintered component still continued to operate within its specifications after 350,000 load cycles despite the higher rated current of 200 A and a higher temperature swing of 110 K, when the test was aborted. Finally, de Monchy discussed the pros and cons of the latest sintering techniques such as the DTF process (Die Transfer Film; Fig. 2; ) and the lamination of wafers with a sintering paste, which was already mentioned in the previous talk.