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10 Tips for Streamlining PCB Thermal Design… A High-Level ‘How To’ Guide

White Paper Many aspects of a PCB’s performance are determined during detailed design, e.g. making a trace a specific length for timing reasons. Timing issues are also affected by temperature differences between...

Engineering Edge

State of the Art Automotive Thermal Design by DENSO

ECUs for vehicles and their components are becoming more complicated and intricate to produce energy-efficient and environmentally friendly cars. Successful thermal design is critical for manufacturers.

By John Parry, Mentor Graphics

DENSO Corporation is a leading automotive supplier that designs and manufactures advanced vehicle control technology, systems, and components for major automotive manufacturers all over the world. Founded in 1949, DENSO is headquartered in Kariya, Japan, operates in 35 countries, and employs around 120,000 people worldwide. DENSO's Electronic Systems Business Group provides engine, transmission, and power management Electronic Control Units (ECUs) as well as semiconductor sensors, integrated circuits, and power modules.

“Our PCB designers use FloTHERM PCB, which has a user-friendly user interface and connection with FloTHERM PACK and we get excellent support from IDAJ and KOZO KEIKAKU ENGINEERING who distribute Mentor Graphics thermal analysis tools”

—Takuya Shinoda, DENSO Corporation

I met up with Takuya Shinoda, Project Assistant Manager of the Technology Planning Department, Electronics Engineering Division 2, to discuss how DENSO are using thermal simulation to dramatically reduce their design time and cost. In this role, he is responsible for the thermal design of Engine Control Units (ECUs). Shinoda has the rare quality of understanding both mechanical and electrical disciplines. As he puts it, “Thermal design bridges both the mechanical and electrical disciplines. Thermal management is mainly a mechanical issue, but heat is generated in silicon, so it is necessary to also understand electronics to do thermal design correctly”.

Design Challenges

ECUs for vehicles and their components are becoming more complicated and intricate to produce energy-efficient and environmentally friendly cars. Successful thermal design is critical for manufacturers.

The junction temperature of the integrated circuits (ICs) or field effect transistors (FETs) that drive such a vehicle system must fall within a guaranteed temperature range. As it has been impossible to directly measure the junction temperature, engineers used to predict electronic elements' junction temperature based on the assumption of the measured surface temperature, and set a wider design margin. In order to cope with the current aggressive price competition, it is important to secure quality, optimize the design margin, and achieve overall cost-effectiveness.

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Figure 1. Downsizing Drives Technology for Heat Dissipation

Simulation Saves Design Time and Cost

Shinoda first started looking for a thermal design tool in 2006, having seen fluid flow visualization of a circuit board for the first time at an exhibition. DENSO selected FloTHERM® and FloTHERM® PCB for their thermal design flow over other tools through a rigorous benchmarking exercise.

Before DENSO started using thermal simulation, a physical ECU prototype had to be created for temperature measurement at the early stage. As a result 2-3 weeks of preparation was needed to perform ½ a day’s testing, which might have to be repeated several times before the product was finalized.

Since 2006, DENSO have consistently increased the use of simulation to reduce the time and cost spent on physical prototyping. By 2009 the ratio of simulation to physical prototyping was 20:80, and this was increased to 50:50 by 2010. By 2012 the ratio stood at 70:30. This change has resulted in a 50% reduction in both the duration and costs associated with thermal design in less than 6 years. DENSO plans to go further, aiming to increase the ratio to over 90:10 by 2015.

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Figure 2. Technological Innovation in Thermal Management

Benefitting from Centralized Thermal Expertise

DENSO has successfully introduced thermal analysis into the manufacturing process by centralizing their thermal technology, and then making this expertise available throughout the company. By listening to the needs of the various design departments, Shinoda’s thermal group is able to quickly improve the quality of design by improving the efficiency of the heat removal using thermal simulation backed up by experiments.

In the thermal design, designers usually focus on changing the case form factors to improve heat dissipation. The best results can be achieved by sharing the case design and electronic design among members of the thermal design team. DENSO had decided to use existing component models (Figure 3). The mechanical team created a smaller case, the circuit design team redesigned 10 to 20 % of the circuit according to new specifications, and the measurement team took the temperature for thermal analysis. As a result of this collaboration, DENSO were able to create a working thermal model for the product in 2 days. Engineers from each team contributed to this effort.

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Figure 3. Technological innovation in thermal management

Characterization Supporting Simulation Accuracy

Besides the aforementioned desire to move away from physical prototyping, measurements have an important role to play in DENSO’s thermal design process. To support their thermal simulations, DENSO uses Mentor Graphics’ T3Ster™ to characterize ECU components and thermal interfaces resistances in situ. The accuracy of the T3Ster data has enabled DENSO to increase the accuracy of their thermal simulations and given them the confident to place such a heavy reliance on simulation.

Measurement of T3ster data is taken back into FloTHERM to improve junction temperature prediction accuracy during design to ensure that junction temperatures never exceed their allowed limits. This is quite a tall order, and requires a very high level of confidence in the simulation models. Today, agreement on junction temperature rise is to within 10% of experiment, and DENSO aim to increase the agreement further, to be within 5% by 2015. "JEDEC JESD51-14 standard was issued in 2010. It has far exceeding accuracy and repeatability compared to the steady-state measurement that conforms to older standards. T3Ster is the only product available in the market that complies with this new standard, enabling the accurate estimation of the thermal resistance and junction temperature. Also, using the structure function, a unique feature of T3Ster, a simple and accurate element model can be generated from the measured data" explained Shinoda.

DENSO has found it indispensable to take accurate measurements from the electronic elements, to improve simulation accuracy, and hence eliminate excess thermal margin from the design.

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Figure 4. Advantages of centralized thermal technology – Simulation & Measurement

The FloTHERM suite of products, including FloTHERM PCB and FloTHERM PACK, has become a major toolset and used across the whole of DENSO’s thermal design process. Backed up by high accuracy package thermal models, material property data and interface resistance values obtained through measurements using T3Ster, Mentor Graphics’ thermal solutions have helped DENSO to achieve over 90% virtualization in their thermal design, and a reduction of more than 50% in both development time and cost, with further saving expected for the future.

“Our PCB designers use FloTHERM PCB, which has a very user-friendly user interface and connection with FloTHERM PACK and we get excellent support from IDAJ and KOZO KEIKAKU ENGINEERING who distribute Mentor Graphics thermal analysis tools,” said Shinoda.

Special Thanks

With thanks to Takuya Shinoda, Project Assistant Manager at DENSO Corporation www.globaldenso.com

 
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