Together with the company’s FloTHERM thermal simulation tool based on computational flow dynamics (CFD), lighting designers can model and improve heat transfer through the LED, printed circuit board (PCB) or luminaire system during design and prototyping phases.

In recent weeks, LEDs Magazine hosted a webcast entitled “Diagnosing and solving thermal challenges in next-generation LEDs,” which detailed this thermal analysis and testing methodology. During the webcast, Boris Marovic, product marketing manager in the Mechanical Analysis Division stated that “excess temperatures are known to cause LED failures in the forms of chip fracture or delamination, phosphor or optics degradation, and decreased light output.”

The need for thermal analysis

Fig. 1.

Before the development of this product, luminaire designers were using thermal-analysis software to accelerate the design of their products, typically based on LED manufacturers’ data sheets. However, the optimization was performed manually. In addition, LED manufacturers’ data sheets are based on the device’s junction temperature, but temperature in the application is often different and must be modeled to fully understand the best way to dissipate heat from luminaires such as street lights, automotive headlights and LED lamps.

"To design our lighting systems, we needed reliable data regarding LED characteristics and simulation tools which deliver results quickly," said András Szalai, chief finanacial officer of HungaroLux of Germany. "We found that Mentor’s T3Ster and FloTherm products were the best such tools available to us and we could use them in our project to verify our PearLight street lighting luminaires for proper heat management. Accuracy and speed in achieving the results was critical for our business."

Using the T3Ster hardware and FlowTherm software simulation, manufacturers have an automated process to optimize their LED or IC package design for effective heat dissipation. Once a device prototype is built, they can characterize the device thermally and build accurate 3-D models in FlowTherm at the subsystem and full system levels. Systems integrators then verify the heat management solution with RC measurements using T3Ster. This set-up is also compatible with EDA and MCAD tools, providing simulation of conduction, convection and radiation to speed the design cycle. The overall process is shown in Fig. 1.

Using T3Ster and FlowTherm

The new product combines thermal and photometric characterization of LEDs, and it can be used as a standalone unit or in combination with the T3Ster tester. The derived thermal model is imported to FlowTherm, which compares simulation results to thermal transient test data. The process allows designers to best design heat sinks, metal-cooled PCBs or luminaire housings to dissipate heat away from the LED environment.

Marovic explained that in thermal transient testing, the LED is powered from one steady state to another using a small forward current. The component will heat up depending on the structure and materials used in the LED and package. The temperature rise and voltage changes provide data for a compact thermal model, a capacitor and resistor model, which is used later in engineering calculations. While such simulation and electrical characterization processes were islands in the past, the T3Ster and FlowTherm products merge the relevant data and simulation.

“If you solve heat flow issues at the LED level, you have not necessarily solved the heat flow issues at the component level. This is also true at the component and system levels,” explained Keith Hanna, director of marketing in the Mechanical Analysis Division of Mentor. Marovic added “Excess temperatures are known to cause LED failures in the forms of chip fracture or delamination, phosphor or optics degradation, and decreased light output.” The new product provides a means of understanding the causes of such failures during a company’s reliability testing, which typically includes temperature shock and aging tests.

The CFD software allows engineers to create virtual prototypes and to simulate air flow, temperature and heat transfer in LED products. Marovic notes that the CFD software is simpler than previous products and users can get up-to-speed with the software with one or two days of training.

“LED manufacturers want to measure the complete product before it ships. Especially for products like automotive headlights, the cost of LED failure is huge, and the headlights must be simulated in a realistic environment because they sit in front of a hot motor as air flows over the subsystem, so manufacturers needs to know that sufficient heat is being dissipated through the entire system,” added Hanna.

The product allows characterization of LED materials. For example, thermal interface materials (TIM) are notoriously difficult to characterize because their conductivity and thickness cannot be determined with high accuracy. By measuring the resistance of TIMs an accurate FlowTherm model can be created that allows product design defects to be identify and manufacturing quality verified. “The ease in creating accurate thermal models based on reliable thermal measurements helps users quickly identify design problems and create design alternatives which improve product quality, reliability and increase profitability,” said Erich Buergel, general manager of Mentor Graphics Mechanical Analysis Division.

In addition, T3Ster is reportedly the only commercially-available thermal characterization tester that fully implements the JEDEC JESD51-14 measurement standard for the junction-to-case thermal resistance of power semiconductor devices. The new test methodology is designed to ensure high accuracy and repeatability compare to steady-state measurements based on older standards. A JEDEC committee is working on a similar standard for LEDs.

Mentor Graphics originally acquired its thermal and photometric simulation capability from Flowmetrics Inc. For more information, see www.mentor.com/mechanical or view the webcast here.