Improving power electronics thermal management and reliability

Driving innovation in thermal management constantly

Zentrum für Wärmemanagement (ZFW) specializes in thermal management research and design services for power electronics for the electric vehicle (EV) sector. They also perform consulting projects for a wide range of applications from battery thermal management systems, advanced driver-assistance system (ADAS) modules and materials characterization to medical device electronics.

Located in Stuttgart, Germany, ZFW’s experienced researchers and design staff are led by Dr. Andreas Griesinger, chief executive officer (CEO), a distinguished authority in the thermal management academic community and professor at the ZFW-affiliated Baden-Wϋrtemberg Co-operative State University.

Established nearly two decades ago, ZFW pushes the boundaries of what is possible to achieve in thermal management, transforming challenges into innovation opportunities. ZFW uses Simcenter™ Micred™ hardware for testing and Simcenter FLOEFD™ software for computational fluid dynamics (CFD) simulation, alongside other Siemens Digital Industries Software tools. Simcenter is part of the Siemens Xcelerator business platform of software, hardware and services. Leveraging Simcenter tools, ZFW helps companies evaluate and optimize thermal management options, so products exceed performance and reliability goals.

A plot of surface temperatures within a power module when mounted to a cold plate.

Enabling design optimization success using simulation and testing

ZFW typically undertakes 150 or more custom projects annually, involving testing, simulation, design or extended research. Ensuring timely response to projects and establishing trust in the quality and accuracy of services provided to clients are key aspects to ZFW’s success.

“Time is often an issue; sometimes, a customer may want their results in a few days,” says Griesinger. “Nothing is standard, most projects are custom. We do the simulation tasks, work on a prototype, complete the test evaluation, optimize and calibrate a model using thermal test data, then we conduct further simulations to reach the optimal design and then assemble a list of recommendations in a comprehensive report for our clients.”

Using Simcenter FLOEFD for CFD simulation enables ZFW engineers to generate thermal models and view results quickly with its ease of use, automatic meshing and guided simulation setup steps. These features benefit staff who need to switch between projects frequently and pick up where they last left off using simulation.

“The significant advantage of using Simcenter FLOEFD is that it is easy to handle and accurate,” says Griesinger. “Other solutions in the market can be far more complicated to use.”

Applying Simcenter Micred test hardware solutions, ZFW can perform accurate, repeatable thermal transient measurements across different projects. These range from determining thermal metrics for semiconductor devices, thermal interface materials (TIMs), light-emitting diodes (LEDs) to evaluating failures and performing lifetime prediction studies for power electronics module reliability tests. Combining thermal measurement and simulation capabilities ensures ZFW generates detailed, accurate information in reports for design decision-making.

Exploring design options fully with easy-to-use CFD simulation

Using Simcenter FLOEFD, ZFW engineers set up simulation models quickly, view and interpret results faster so they can focus on value adding design optimization. Working with clients who provide models from various computer-aided design (CAD) tools, ZFW takes advantage of robust handling of complex CAD geometry and automatic meshing in Simcenter FLOEFD. Preparing complex CAD assembly models for CFD analysis requires minimal preprocessing steps.

In various projects, ZFW explored many design variations and operating points efficiently using what-if analysis parametric studies. They also combine Simcenter FLOEFD with Simcenter HEEDS™ design exploration software to evaluate large, multiparameter design sweeps. By leveraging the SHERPA direct optimization algorithm in Simcenter HEEDS with fast, accurate CFD simulation, ZFW can perform extensive studies for liquid cooling design optimization. For example, cold plate flow channel patterns can be optimized for thermal performance and minimized pressure drop.

Furthermore, at the ZFW affiliated Baden-Wϋrtemberg University, CFD simulation is part of the curriculum for teaching and projects. “Students use Simcenter FLOEFD to create models and perform calculations,” says Griesinger. “You can use Simcenter FLOEFD after only a few hours of familiarization. This ease of use is a great advantage.”

Understanding the heat flow path in real-world electronic systems

How heat dissipates from within a semiconductor device integrated into a system is difficult to discern using many typical test solutions. For example, infrared (IR) cameras are widely used to see surface temperature variations and hotspots; however, there are limitations for accurately revealing internal structure thermal influences. There are various measurement options for characterizing individual materials such as TIMs standalone. In physical electronic systems, integration and manufacturing influences also have an impact on thermal behavior. For example, contact thermal resistance between layers and interfaces can vary due to surface finish and pressure variations.

Using a combination of transient thermal measurement and 3D thermal simulation, ZFW can fully evaluate underlying influences on thermal behavior for electronic devices in systems to make informed recommendations on design improvement.

“The advantage of using Simcenter Micred T3STER is that you get thermal data from a real-world system,” says Griesinger. “You can get information about the complete heat flow path, so you can see contributions of the internal structure of the device, the TIM material, a PCB, to an aluminum cold pate or heat sink and out to the cooling fluid, essentially all influences of materials, layers and interfaces from one measurement.”

The core of Simcenter Micred T3STER™ hardware technology is accurately measuring junction temperature via the electrical test method. Precisely measuring the thermal response to an applied power step on a device enables capture of a thermal impedance curve. A structure function is generated from this, which is a plot of thermal resistance versus thermal heat capacitance, representing the heat flow path from device junction to ambient. From this you can extract thermal resistance metrics, identify defects, track thermal degradation and calibrate thermal models.

“There is value in the software that comes with Simcenter Micred T3STER hardware,” says Griesinger. “We are happy with the accuracy of structure functions, which we use to understand different thermal resistances.”

Performing accurate connector thermal characterization

There are various uses that ZFW applies Simcenter Micred T3STER hardware and structure function analysis to, including connector thermal characterization for vehicle and high-power applications. With custom test rigs to support thermal impedance measurements, researchers often look at two types of generated structure function (cumulative and derivative) for studies. In one study, researchers used structure functions to evaluate the interfaces of the connector terminals, housing and sensor. They easily identified contributions of contact thermal resistances and TIM material under pressure from inserting a plug terminal. ZFW staff accurately established thermal resistance values representing real-world use.

Thermal characterization testing of a connector at ZFW.

Assessing inverter liquid cooling performance

To validate an inverter’s liquid cooling design, it is beneficial to closely assess the fluid mechanics performance (pressure drop) and thermal heat dissipation characteristics from within the module. Whether a typical cold plate or advanced dual-sided cooling arrangement is used, proper thermal management of critical power semiconductors ensures reliable performance over the inverter’s intended lifetime. ZFW has extensive expertise creating custom test arrangements for the vehicle electrification and wider power electronics sectors.

Using Simcenter Micred T3STER or Simcenter Micred Power Tester hardware for non-intrusive thermal measurement in conjunction with a fluid mechanics testing rig, ZFW determines precise thermal resistance data of the heat flow path from the power semiconductor to coolant. This is alongside corresponding flow rate, pressure drop and coolant temperature difference between the cold plate’s inlet and outlet. ZFW can easily extract thermal resistance metrics by viewing and comparing structure functions generated from thermal transient measurements.

“Leveraging Simcenter Micred T3STER gives us precise data on the heat flow path from the junction source to the cooling liquid, so we get an accurate localized measurement of thermal resistance,” says Griesinger.

A power module thermal transient measurement (thermal impedance curve shown) and generated structure functions for determining thermal resistance values for different flow rates, from “New Measurement Method for Direct Cooled Power Electronic with Transmission Fluids” published in the Journal of Electronics Cooling and Thermal Control, Vol.10 No.1, March 2021.

Improving thermal reliability using power cycling tests

ZFW uses the Simcenter Micred Power Tester for its combined power cycling and structure function sampling capability, which provides failure and degradation insights without removing a device or module from its fixture during testing. Power cycling testing is increasingly a thermal reliability assessment method for demanding power electronics applications and features in the automotive ECPE AQG 324 guidelines.

ZFW applies Simcenter Micred Power Tester to projects where clients request inverter modules are subjected to an accelerated thermal reliability test to predict lifetime of crucial power semiconductors. Clients want to find root causes of failure to identify opportunities for design improvement. Additionally, ZFW undertakes detailed studies on TIMs to evaluate various samples, surface finish influences and thermal cycling degradation impacts on material properties.

In one laser module project, combining power cycling and evaluating the thermal structure using structure functions proved essential for identifying the root cause of failure. Rapid transient heating in the initial seconds of each power cycle contributed to accelerated degradation, leading to early failure of the module (in between 10 to 20 thousand cycles). ZFW confidently recommended a design change to the heat slug (heat spreader) within the thermal structure to smooth the module’s heating profile.

“Using Simcenter Micred Power Tester, our researchers identified the cause of a module’s early failure and implemented a design change to the thermal structure, resulting in an improved lifetime by a factor of 10,” says Griesinger.

This modification significantly increased the number of operational cycles the power module could withstand, indicating a lifetime of between 10 and 20 million cycles. Without power cycling testing, such root cause insights are not possible.

Leveraging a simulation and testdriven approach for future challenges

ZFW has developed a proven approach for employing simulation and testing to solve difficult challenges in thermal management of electronics.

“We’ve used Siemens’ Simcenter tools successfully for over 20 years,” states Griesinger. “These thermal test and simulation solutions continue to enable us to serve our clients’ needs fully.”