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WP23 How to Measure Thermal Resistance of LED Emitters and LED Arrays White Paper 20161201 ©2016 Lumileds Holding B.V. All rights reserved. White PaPer how to Measure thermal resistance of LED Emitters and LED Arrays Purpose To describe the process used to determine typical thermal resistance values from the LED Junction to Case and LED Junction to Ts reference point. Introduction Thermal performance is the most critical factor of a well-designed LED lighting system. A lighting system with proper thermal design has higher efficacy, meaning more light can be extracted using less energy, and better long term reliability. Thermal resistance, R th, is defined as the ratio of the change in device temperature, ΔT, to the electrical power applied, P, and is expressed in °C/W or K/W. That is, The thermal resistance between LED junction and case, Rth J-C, is an important device parameter for understanding the thermal performance limit when an LED is in good contact with a properly dimensioned heat sink. Understanding the thermal resistance of the LED device from junction to case is essential when designing a lighting system for maximum thermal performance. Lumileds provides the R th J-C value for each LUXEON device in it’s respective product datasheet. The thermal resistance between junction and sensor pad, Rth J-S, is important for determining and monitoring the LED junction temperature in situ. Understanding Rth J-S is important for design validation and in some applications can be used to limit the LED junction temperature. Lumileds provides the Rth J-S value for each LUXEON device in its respective application brief. It is important to note that all R th values listed in LUXEON Device datasheets and application briefs are based on the electrical power going into the LED.

WP23 How to Measure Thermal Resistance of LED Emitters and LED Arrays White Paper 20161201 ©2016 Lumileds Holding B.V. All rights reserved. 4 Determining K-factor The MicReD system is designed to measure small voltage changes in a semiconductor device during operation over time with great accuracy and speed. In order to translate this direct measurement into a thermal resistance measurement, the voltage change must first be converted to a temperature change using a calibration factor, often referred to as the “K-factor.” To measure the K-factor, the reliability board is heated to a particular temperature, such as 30°C, and then the voltage to each device is measured in response to a small sense current, IS, (small enough to not result in further device heating, typically about 1/50 of the nominal device current). The board is then heated to another temperature, say 45°C, and the measurements are repeated. This process happens for several temperatures up to 90°C and then the K-factor is calculated for each device and saved in the MicReD system. As the device should not be heated by the sense current, the temperature of the reliability board can be assumed to be equal to the junction temperature, T Testing for Thermal Resistance Junction to Case Once the K-factor is established, the reliability board is ready for measuring the transient cooling curves. A heating current, IH (typically the nominal current of the device) is applied to the device until the device has reached a hot steady-state. At this condition, voltage, VH, is recorded and then IH is switched off. Then, the same sensing current which was used for determining the K-factor is used again to measure the voltage as a function of time as the part subsequently cools to a cool steady-state condition. The measured voltage change vs. time curves are then converted into temperature change vs. time using the pre- determined K-factor by the T3ster software. The thermal impedance curves, Z th(t) are then determined from the temperature change (the temperature at time, t, minus the temperature at t = 0 when the heating curve was switched off) divided by the heating power. That is, These curves are recorded twice for each device using two very different TIMs. The resulting change in the contact resistance due to the different TIMs alters the total thermal resistance (often referred to as junction-to-heat-sink or junction-to-ambient thermal resistance) under steady-state conditions and therefore for a single device, the thermal impedance curves from the two measurements will follow each other from the junction to the point where the additional contact resistance occurs, after which the curves will diverge. This divergence point is recognized as the case/TIM interface and the thermal impedance at this point is approximately equal to the steady-state Rth J-C. Determining Rth J-C It is often difficult to determine the divergence point of thermal impedance curves and therefore further mathematical transformations of the impedance curves are required to make the difference more noticeable in order to determine the R th J-C. Thermal impedance curves or Zth(t) must be transformed into structure functions using a series of derivations and de-convolutions (the details of which can be found in the appendices of JEDEC Standard JESD51-14). The T3ster software produces two types of structure function curves: cumulative and differential. The cumulative structure function is the cumulative thermal capacitance plotted against the cumulative thermal resistance from the junction of the device. The differential structure function, on the other hand, is the derivative of the cumulative thermal capacitance plotted against the cumulative thermal resistance. In a cumulative structure function curve, a difference in material properties is observed as a change in slope. On a differential structure function curve, a change in material properties is observed as a peak.

WP23 How to Measure Thermal Resistance of LED Emitters and LED Arrays White Paper 20161201 ©2016 Lumileds Holding B.V. All rights reserved. 5 In a simple system, it may be possible to determine the thermal resistance of each material component by observing the peaks or slope-changes on the structure function curves. However, caution should be used when using peaks or slope changes to determine the thermal resistance of a more complex device, such as an LED. Again, it is recommended to follow JEDEC Standard JESD51-14 for determining thermal resistance and use the divergence point of the structure function curves of a device tested with two different TIMs to determine the junction-to-stage thermal resistance value. In general, it is difficult to mathematically determine the junction-to-case thermal resistance of a particular device using the divergence point of two curves; therefore the structure function curves must be analyzed by a skilled scientist/engineer. Experience has shown that the divergence point can be more easily determined from differential structure functions; however, cumulative structure functions are also examined to confirm the results inferred from the differential structure functions and, in some cases, to better pinpoint the thermal resistance. The method for determining the thermal resistance junction-to-case is the same whether examining either type of structure function curves, and so for simplicity only the differential structure function analysis is shown below. The structure functions are extracted from the T3ster software and plotted in a graphing package of choice. Care must be taken to correctly label each curve with the corresponding part and TIM for each measurement. The differential structure function curves are plotted on a log10 derivative of thermal capacitance vs. thermal resistance chart (see Figure 6a). The point on the x-axis where the curves asymptote vertically is the total thermal resistance (junction-to-heat-sink/ambient). The divergence point is better observed by narrowing the scale to “zoom-in” on the region of interest (see Figure 6b). The error can further be reduced by ensuring the lines and/or symbols used to display the graph are as thin/small as possible. Once the details of the curves are clearly visible, look for the divergence points of the individual parts and read the R th J-C value from the x-axis at that point of divergence. For a large group of parts which may prove too time consuming to determine individual Rth J-C values for each TIM pair, it may make more sense to look at the group of parts as a whole. Figure 7 shows a large group of parts measured with two TIMs. In this case, the thermal resistance for the group can be approximated by looking at the typical divergence point of the group with errors added to either side representative of the maximum and minimum Rth J-C of the group’s divergence. In Figure 7, the Rth J-C was determined to be 0.49 ±0.05 K/W. It is important to point out that the types of TIMs used should not affect the Rth J-C determined from the divergence point of the structure functions. For example, in Figure 8, differential structure functions are plotted for a single device measured with 3 different TIMs. The total thermal resistance varies, as expected; however, the divergence point of these curves (indicating the Rth J-C ) is unchanged no matter which TIM is chosen. Furthermore, after observing the shapes of these curves it becomes clear how difficult it would be to determine the thermal resistances of individual components corresponding to each individual peak for this complex system. Therefore, R th J-C is best determined by the divergence point of the structure functions for parts measured using two different TIMs.

junction temperature can be calculated. important to use the same TIM called out in the respective product’s application brief to achieve consistent measurement results. or is a part of the thermal pad design of the PCB to which the device is attached. To determine the Rth J-S, a 40 gauge Type K thermocouple is attached at the designated point with Artic Silver Epoxy (see Figure 9). in which the devices are attached to the reliability board with the thermal conductive TIM and not with the Kapton tape insulator. vary dependent upon the thermal interface material chosen. Figure 9. Example of thermocouple attachment to LUXEON MZ Starboard on the left and to a LUXEON CoB array on the right.

To calculate or determine the Rth J-S, three data points are necessary.

1. Electrical Power through the device in Watts
2. LED junction temperature derived from T3ster Data.
3. Temperature at the Ts point.

Thermal resistance is equal to the change in temperature divided by the power applied. to run a scripted system control program with Agilent VEE Pro. establishing the K-factor as described in the Rth J-C measurement process. Junction temperature is calculated from the steady state forward voltage using the slope defined by the short pulse K-Factor. ambient and LED junction to solder pad using the formula above. Figure 10. Input signal vs. junction temperature.

WP23 How to Measure Thermal Resistance of LED Emitters and LED Arrays White Paper 20161201 ©2016 Lumileds Holding B.V. All rights reserved. 9 Conclusion The thermal resistance values Rth J-C and Rth J-S are essential to develop and monitor the thermal performance of LED Lighting Systems.

• Rth J-C defines the ability of the LED package to dissipate heat from the device junction to the case of the package.
• Rth J-C is a critical factor necessary for designing a proper thermal system.
• Rth J-C is a constant, unaffected by TIM or heat sink selection.
• Rth J-S defines the thermal resistance between a sensor point near the direct thermal path allowing for monitoring of the LED junction temperature for the purpose of design validation or system performance monitoring.
• Rth J-S is defined for a specific LED, board, and TIM configuration. Changing the board design and/or TIM will affect its value. References 1. MIL Standard 833: MIL-STD-883E, METHOD 1012.1, Thermal Characteristics of Integrated Circuits, 4 November 1980. (http://www.thermengr.net/PDF/MilStd883M1012_IC.pdf) 2. JEDEC Standard JESD51-14: JEDEC STANDARD, JESD51-14, Transient Dual Interface Test Method for the Measurement of the Thermal Resistance Junction to Case of Semiconductor Devices with Heat Flow Through a Single Path, November 2010. (www.jedec.org/sites/default/files/docs/JESD51-14.pdf) 3. MicReD® and T3ster®: For more information on these systems, please refer to www.mentor.com/micred.

©2016 Lumileds Holding B.V. All rights reserved. LUXEON is a registered trademark of the Lumileds Holding B.V. in the United States and other countries. lumileds.com Neither Lumileds Holding B.V. nor its affiliates shall be liable for any kind of loss of data or any other damages, direct, indirect or consequential, resulting from the use of the provided information and data. Although Lumileds Holding B.V. and/or its affiliates have attempted to provide the most accurate information and data, the materials and services information and data are provided “as is,” and neither Lumileds Holding B.V. nor its affiliates warrants or guarantees the contents and correctness of the provided information and data. Lumileds Holding B.V. and its affiliates reserve the right to make changes without notice. You as user agree to this disclaimer and user agreement with the download or use of the provided materials, information and data. WP23 How to Measure Thermal Resistance of LED Emitters and LED Arrays White Paper 20161201 About Lumileds Lumileds is the global leader in light engine technology. The company develops, manufactures and distributes groundbreaking LEDs and automotive lighting products that shatter the status quo and help customers gain and maintain a competitive edge. With a rich history of industry “firsts,” Lumileds is uniquely positioned to deliver lighting advancements well into the future by maintaining an unwavering focus on quality, innovation and reliability. To learn more about our portfolio of light engines, visit lumileds.com.