Special circumstances

Every development project, every major program is facing its unique challenges and tough moments. None of the seven primary employees of SwissSEM meeting back then in December 2019 in Shanghai would have expected that this should be the last time that we met in person for more than 2.5 yrs now. We had extremely ambitious plans - developing our own chip-set with a foundry partner, erecting a factory where there is literally a green field, filling the factory with tools for power module assembly and meanwhile generating a supply base with collaboration partners for all the raw-materials you need.

We are grateful and thank our partners and suppliers for their trust in our common journey. Thank you!

A long list of complex and interconnected projects with unpredictable difficulties that we were not always grateful for were mastered by facing the issues, working on solutions by technical excellence, identifying pragmatic approaches, respect and open speech and humility. We experienced the team-spirit when celebrating the Opening Ceremony, virtually attended our contributions to conferences and felt the progress.

That we have such a high performing product qualified and a leading edge factory generated in less than 30 months is due to our common efforts.

ED-Type module design optimization

As an emerging company it is crucial for SwissSEM Technologies AG to bring its first products to the market in high quality and short time. Electromagnetic and thermal optimization are essential for excellent device performance. The ED-Type, an industry standard 17 mm height 62 x 152 mm IGBT module, offers special challenges for internal current sharing between the IGBTs due to its longish design. Most classic layouts suffer more or less current imbalance between the chips, and it is our goal to launch a module with the best possible current homogeneity in order to have the full benefit of our latest IGBT i20 generation.

With the help of the MFis Wire software we were able to quickly generate various design variants including variations in the bond wire layout. This enabled us to simulate the electromagnetic couplings of the variants in Q3D and make switching simulations with the SIMetrix Spice simulator using the extracted circuit models from Q3D. These simulations were the basis for a better understanding of the device and its internal couplings. Especially as already small variations of the wire position and shape in the mm range can have a significant impact on the coupling. Hence a simplified geometry, as it would be obtained when using the wire tool available in Q3D, is not sufficient. Together with heat-transfer simulations an optimized layout was found.

From thermal resistance point of view both variants of chip positioning offer the same Rth. However, the “Layout straight” offers more potential to improve the current sharing compared to the “Layout classic”, especially to slow down IGBT #3 which is closest to common power emitter connection (see figure 1). For the final layout optimization, the gate-position of IGBT #3 was rotated and the main emitter wire and gate wire layout was optimized (see figure 2). As a result, the current imbalance was reduced from 30% of the “Classic layout” to 17% of the “Layout straight optimized”. This is a significant step that improves the load balancing within the IGBTs, but as well yields in a higher safe operating area utilization of the IGBT chips.

Today’s simulation tools for thermal as well as electro-magnetic simulations are very powerful, shorten development time and improve the quality of IGBT module designs significantly. Still the input for the finite element simulations need to be as accurate as possible and reflect the final product design if the optimal result is to be achieved. Especially for complex details like wire bonds, obvious simplifications are attractive at first glance due to the tedious and time-consuming work it requires in the CAD. However, the accuracy of the results will suffer from simplifications, and the full potential of the simulation tools is not utilized.

By using the software MFis Wire, time is significantly shortened for creating complex 3D geometry models of bond wire layouts. Using hexagonal wire cross section in the input geometry of the parasitic extractor results in four times faster computation, which makes it possible to investigate several layout variants in a single workday. This method used at SwissSEM, enabled an improvement of the ED-Type module’s internal current sharing by nearly a factor of two compared to classic design approaches.