iMaps Announcements 02/2025

Don't miss the IMAPS spring seminar!

The spring seminar on "Reliable electronics - made in Europe" is just around the corner. This time, the German IMAPS community will meet on February 25, 2025 at the Fraunhofer Institute for Microstructure of Materials and Systems IMWS in Halle an der Saale. You can be there too!

Here is the overview of the lecture program:

Andreas Karch (Indium Corporation), Modern high-reliability solder paste system for applications with sustainable long mission profiles

Kurt Jürgen Lang (ams-Osram), Solderpad design and processing for reliable LED applications

Oliver Albrecht (TU Dresden / IAVT), In-situ X-ray methods for the characterization of soldering processes and thermomechanical processes

Bernhard Wunderle (TU Chemnitz), Thermo-Mechanical Reliability for Electronics Packaging

Martin Rittner (Robert Bosch GmbH), Robust power module technologies: Challenge for Module Qualification

Robert Klengel (Fraunhofer IMWS), Accelerated reliability assessment for base plate metallization of power modules

Markus Leicht (Schaeffler), Power-Cycling, Livetime modeling based on separation of microstructural degradation mechanisms and their interrelation with electrical functionality

Stefan Wagner (Fraunhofer IZM), Thermally and moisture driven failures in the application of power electronic systems

Christoph Hecht (FAPS), Near-chip 3D functionalization of DCB substrates for power electronic applications

Ralph Schacht (BTU), Transient system level simulation - model development and experimental validation of the online junction temperature

Johannes Zeh (CiS), Reliability of sensor signals

Jens Müller (TU Ilmenau), LTCC and LTCC composite substrates for sensor applications under harsh operating conditions

The registration page, more detailed information on the seminar and details of the venue can be found at

www.conftool.net/imaps-fruehjahrsseminar-2025/index.php and https://imaps.de/events/.

Winner of the Best Presentation Award

We are pleased to give the winner of the "Best Presentation Award" of this year's IMAPS Autumn Conference, Dr. Freerik Forndran (Schaeffler AG), the opportunity to present his work here.

Physics-of-Failure based lifetime modeling of silver sintered power modules for electric vehicles by experiment and simulation

The paradigm shift in automotive power electronics towards wide-bandgap semiconductor devices such as silicon carbide (SiC) MOSFETs brings fundamental challenges, especially with regard to lifetime and reliability evaluations. Wide-bandgap materials offer advantages such as higher efficiency and temperature resistance, but also require new approaches in packaging technology (AVT). In particular, the use of sintered silver interconnect layers (SAG) for near-chip AVT has proven to be promising. This technology is characterized in particular by its excellent electrical, thermal and mechanical properties, which are mainly determined by the porous microstructure. However, the use of new packaging solutions means that the empirical lifetime models for power modules developed over many years are no longer suitable. A holistic physics-of-failure approach can provide a remedy here, as it enables a significant reduction in test time through finite element simulations. This approach requires a detailed understanding of the relevant failure mechanisms as well as an electrical, thermal and mechanical characterization of the materials involved.

Fig. 1: LEFT: Exemplary image of an AMB substrate with four assembled SiC MOSFETs and the top-side connection consisting of copper foil and ribbon bonds. RIGHT: Schematic representation of the investigated module in cross-section (not to scale)

The aim of this work was to use the physics-of-failure approach to create a service life model for a complex power module for electric vehicles. The module under investigation is shown in Figure 1. It consists of a ceramic substrate (referred to here as AMB substrate), consisting of two copper and one ceramic layer, which is attached to a cooling structure (cooler). A silicon carbide (SiC) MOSFET is sintered onto the substrate. The top side of the semiconductor component is connected via a sintered copper foil and copper ribbon bonds. In order to investigate application-relevant degradation behavior, the power modules are subjected to so-called power cycling experiments. Here, the semiconductor components are periodically energized and the generated losses lead to a cyclical temperature increase and thus to a thermo-mechanical load [1]. In these experiments, the top-side sintered connection between the semiconductor device and the copper foil proved to be particularly susceptible to degradation.

The identification of degradation of the top-side bonding layer of sintered silver as the dominant failure mechanism determines the further procedure within the framework of the physics-of-failure approach. A detailed electrical, thermal and mechanical characterization of the sintered silver in particular is therefore necessary for the following realistic simulative description of the power module and the degradation.

Tensile tests are a common method for determining mechanical material data. However, special attention must be paid to the microstructure when producing the tensile specimens (shown in Fig. 2a). This should correspond to the microstructure of the real bonding layer, as the porosity has a dominant influence on the material properties. To achieve this, the process had to be adapted and this match was ensured by means of initial microsection tests. Classical uniaxial tensile tests were used to obtain the elasto-plastic material parameters. A creep test was also carried out to investigate the time-dependent behavior. It has been state of the art for several years that sintered silver exhibits pronounced creep at temperatures lower than a homologous temperature of 0.3. This is due, among other things, to the stress increases that occur due to the reduced cross-sectional area of the porous material. In addition, the tensile specimen geometry can be used for the electrical and thermal characterization of the sintered silver. Literature models were used to model the measured elasto-plastic data. The creep curves were characterized by a strong temperature and stress dependence, and no literature model could be found that was able to describe the data. Therefore, an in-house model was developed based on the data, which takes into account both primary and secondary creep. As shown in Figure 2a, the developed model describes the measured data well for all temperatures and stresses. The model was implemented into the finite element (FE) software via a subroutine. For further information, please refer to [2].

Thus, for the first time, a holistic creep model was used in this work, which depicts primary and secondary creep. The following service life simulations were designed as multidomain simulations: An electro-thermal simulation calculated the temperature distribution following from the current, taking into account the feedback due to the temperature-dependent resistance, then the resulting temperature distribution was used as an input variable for a thermo-mechanical simulation in order to determine and evaluate the stress and strain states. For the service life model to be created, a scalar failure parameter was also required, which resulted from averaging the inelastic (plastic + creep) strain along a representative crack path [3]. The required experimental failure data came from three power cycling experiments with different loading scenarios. The resulting fatigue life model is shown in Figure 2b. The dependency could be confirmed with a fit according to the Coffin-Manson model. Such a calibrated physics-of-failure service life model now makes it possible to make meaningful reliability predictions for deviating loads or modified geometries using FE simulations only. In summary: In this work, a first physics-of-failure-based service life model was designed for a complex power module for electric vehicles. The module under investigation was characterized by the use of SiC MOSFETs as power switches and sintered silver in both the bottom and top connections. The necessary material characterization, especially of sintered silver, was carried out by means of tensile tests and a special creep model was developed to describe the measured data. In the future, the interaction of simulation and experiment as part of a physics-of-failure approach will allow test times to be drastically reduced.

Fig. 2: (a) Image of a tensile specimen and comparison of measured creep data and developed model. (b) Generated service life model according to the physics-of-failure approach

IMAPS Germany - Your Association for Assembly and Connection Technology

IMAPS Germany, part of the "International Microelectronics and Packaging Society" (IMAPS), has been the forum in Germany for all those involved in microelectronics and packaging technology since 1973. With almost 300 members, we essentially pursue three important goals:

• we connect science and practice
• we ensure the exchange of information among our members and
• we represent the position of our members in international committees.

Calendar of events

References

[1] European Center for Power Electronics (ECPE), Qualification of Power Modules for Use in Power Electronics Converter Units in Motor Vehicles, Guideline AQG 324 V03.1/2021, 2021.
[2] F. Forndran, Physics-of-Failure Based Lifetime Modeling of Silver Sintered Power Modules for Electric Vehicles by Experiment and Simulation, Chemnitz University of Technology, 2024.
[3] B. Wunderle, K.-F. Becker, R. Sinning, O. Wittler, R. Schacht, H. Walter, M. Schneider-Ramelow, K. Halser, N. Simper, B. Michel and H. Reichl, "Thermo-mechanical reliability during technology development of power chip-on-board assemblies with encapsulation," Microsystem Technologies, vol. 15, pp. 1467-1478, 2009.

Contact:

Dr. Freerik Forndran
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Imprint

IMAPS Germany e. V.
Kleingrötzing 1, D-84494 Neumarkt-St. Veit

1st Chairman: Prof. Dr.-Ing. Martin Schneider-Ramelow, Director of the Fraunhofer Institute for Reliability and Microintegration (IZM),
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Treasurer
(for questions about membership and contributions):
Ernst G. M. Eggelaar, This email address is being protected from spambots. You need JavaScript enabled to view it.

You can find detailed contact information for the members of the Executive Board at www.imaps.de
(Executive Board)