Optimizing PCB Thermal Performance for Cree® XLamp® XQ & XH Family LEDs Table of Contents Introduction Introduction....................................................................................1 This application note outlines how to design a thermally Background....................................................................................2 effective printed circuit board (PCB) to use with the XQ and XH Thermal Simulation........................................................................2 families of Cree XLamp® LEDs. Thermal Spreading.........................................................................3 Thermal Crosstalk..........................................................................3 Copper Trace Size..........................................................................5 Usefulness of Thermal Vias..........................................................6 FR‑4 Board Thickness....................................................................8 FR‑4 Trace Thickness.....................................................................8 MCPCB Trace Thickness...............................................................9 MCPCB Dielectric Thermal Conductivity......................................9 www.cree.com/Xlamp One of the most critical design parameters for an LED‑based illumination system is its ability to conduct heat away from the LED junction. High operating temperatures at the LED junction adversely affect the performance of LEDs, resulting in decreased light output and lifetime.1 Specific practices should be followed in the design, assembly and operation of LEDs in lighting applications to properly manage this heat. FR-4 vs. MCPCB...........................................................................10 This application note provides guidelines for designing a PCB Measured vs. Simulated Results.................................................10 layout that optimizes heat transfer from XQ and XH family LEDs. Trace Size Recommendations....................................................11 Guidelines are provided for FR-4 and metal‑core printed‑circuit FR-4 PCB with XQ Family LED..............................................11 boards (MCPCB). Cree encourages its customers to consider MCPCB with XQ Family LED................................................12 these guidelines when evaluating the many LED thermal FR-4 with XH Family LED......................................................12 management techniques available. For additional guidelines on MCPCB with XH Family LED................................................12 LED thermal management, refer to the Thermal Management Chemical Compatibility...............................................................12 CLD-AP167 Rev 0D Technical Article application note. 1 See the Long-Term Lumen Maintenance application note Copyright © 2013-2016 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree® and XLamp® are registered trademarks and the Cree logo is a trademark of Cree, Inc. This document is provided for informational purposes only and is not a warranty or a specification. For product specifications, please see the data sheets available at www.cree.com. For warranty information, please contact Cree Sales at [email protected]. Cree, Inc. 4600 Silicon Drive Durham, NC 27703 USA Tel: +1.919.313.5300 1 Optimizing XQ & XH PCB Thermal Performance Background Prior to the introduction of the XQ and XH families of LEDs, Cree XLamp LEDs were designed with an electrically isolated thermal path. The neutral thermal pad simplifies the design of PCBs for thermal considerations. The pad provides a path for heat transfer away from the LED chip junction to the thermal pad. Being electrically isolated from the anode and cathode of the LED means the pad can be soldered Isolated thermal pad or attached directly to the PCB or heat sink. With a neutral thermal pad, thermal vias can be placed directly under the thermal pad, but because they have electrically conductive pads, this cannot be done for XQ and XH family LEDs. Figure 1 shows this difference. + - Copper Plane Copper Thermal Vias + - Copper Plane Copper Thermal Vias Shorting together Figure 1: LEDs with electrically isolated (top) and non-isolated (bottom) thermal pads mounted on a circuit board Thermal Simulation Non-isolated The data presented here are the results of simulations conducted using ANSYS DesignSpace2 and Autodesk Simulation CFD3 software. A standard off‑the‑shelf heat sink4 with a pre‑attached thermal interface material (TIM) was used for all the simulations, with convection set to 7 W/m2 °C and 1 W of total input for all scenarios. Notes: • Actual results may vary with different geometries and materials. Cree recommends performing actual verification testing to validate the thermal management of any LED‑based illumination system. Cree’s Thermal, Electrical, Mechanical, Photometric and Optical (TEMPO) tests such as TEMPO 24 or TEMPO-SPOT Thermal can help with this verification. 2 3 4 ANSYS, Inc., www.ansys.com/Products/Simulation+Technology/Structural+Mechanics/ANSYS+DesignSpace Autodesk Inc., www.autodesk.com/products/autodesk-simulation-family/features/simulation-cfd Model 374424B00035G, Aavid Thermalloy, www.aavid.com/products/bga/374424b00035g Copyright © 2013-2016 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree® and XLamp® are registered trademarks and the Cree logo is a trademark of Cree, Inc. This document is provided for informational purposes only and is not a warranty or a specification. For product specifications, please see the data sheets available at www.cree.com. For warranty information, please contact Cree Sales at [email protected]. 2 Optimizing XQ & XH PCB Thermal Performance • Modeling and simulation are attempts to predict future performance based on assumptions and criteria that may differ from actual device use and environment and slight modifications must sometimes be made to the design to enable the modeling and simulation process. Actual results may differ from the modeling and simulation results due to these modifications and actual device use and environment. Thermal Spreading We simulated thermal spreading to determine if it is proportional to the size of the contact pads of the XQ and XH family LEDs. If the thermal spreading is not proportional, the traces for each contact pad must be sized to account for the disproportionality. Figure 2 shows simulated thermal spreading for a single XQ-D LED on a 3.3 mm X 3.3 mm trace and a single XH-G LED on a 5.0 mm X 5.0 mm trace, both mounted on an MCPCB with 2‑oz thick copper and a dielectric thermal conductivity of 2.2 W/mK. The XQ-D LED contact pads are of equal size and the proportion of simulated thermal flux conducted through each pad is within 0.3% of being equal. There is a 68.3% to 31.7% size difference in the contact pads of the XH-G LED and the proportion of thermal flux conducted through each is within 3.7% of ding from XQ being the same (64.6% to 35.4%). In each case, the proportion of thermal flux conducted through each contact pad is in direct correlation with the relative sizes of the pads. Therefore it is logical to scale the sizes of the traces to be in proportion to the sizes of the contact pads. on small trace on MCPCB Thermal spreading from XH • Single XH-G on small trace on MCPCB s Objective: Show how much heat flows through each contact pad x 49.7% of total heatheat flux 49.7% of total flux this through this pad through contact pad 50.3% 50.3%ofoftotal totalheat heat flux Conclusion: fluxheat through this padpad More flows through larger through this contact 64.6% flux 64.6%ofoftotal total heat heat flux through throughlarge largecontact contact pad pad pad, roughly proportional to 35.4% flux 35.4%of oftotal total heat heat flux through pad throughsmall small contact contact pad the 2: size differential Figure Thermal spreading simulation for XQ-D (left) and XH-G (right) LEDs Area: 4.23mm2 for large, 1.96mm2 for small (68.3/31.7%) 4 Thermal Crosstalk Copyright © 2013, Cree Inc. 18 Thermal crosstalk occurs when the heat output from one LED affects an adjacent LED. We compared the thermal resistance, junction to ambient temperature (Θj-a) of the center LED in a 3 X 3 pattern of nine LEDs mounted on a 50 mm X 50 mm PCB on an arbitrary sink with a 3.3 mm X 3.3 mm trace to determine how closely together XQ family LEDs can be located without incurring a large thermal crosstalk penalty. For XH family LEDs, we used a 5 mm X 5 mm trace. Copyright © 2013-2016 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree® and XLamp® are registered trademarks and the Cree logo is a trademark of Cree, Inc. This document is provided for informational purposes only and is not a warranty or a specification. For product specifications, please see the data sheets available at www.cree.com. For warranty information, please contact Cree Sales at [email protected]. 3 9x Optimizing XQ & XH PCB Thermal Performance arbitrary heat sink with a 5mm trace Comparison of the j-a of the center• LED each condition Figure 3for shows the inter‑LED spacing measurements. Comparison of the j-a of the center LED for each condition (w 110.0% 110. 105.0% 100.0% 95.0% % % _j-a of 10mm spacing • 9x XQ-D on a 50x50mm PCB on an • arbitrary heat sink with a small trace _j-a of 10mm spacing • without a huge penalty in thermal cross-talk w Conclusion: C Very minimal ‘cross-talk’ for MCPCB. V More, still small forPCB FR4 even as close as 2mm XH-D on but a 50x50mm on an M 90.0% 0 2 105. 100. 95. 10 90. 4 6 8 Distance between LEDs (mm) Figure 3: Inter‑LED spacing for thermal crosstalk simulation for XQ-D (left) and XH-G (right) LEDs Figure 4 and Figure 5 show thermal simulation images for various inter‑LED spacings, defined as the gap between the LEDs. 10mm 2‑mm spacing Copyright © 2013, Cree Inc. 2.3‑mm spacing 3‑mm spacing 7mm 5‑mm spacing 5mm 10mm 3mm 7mm 7‑mm spacing 10‑mm spacing 2mm 5mm Figure 4: Simulation of XQ-D thermal crosstalk Copyright © 2013, Cree Inc. 3‑mm spacing 5‑mm spacing 7‑mm spacing 10‑mm spacing Figure 5: Simulation of XH-G thermal crosstalk Copyright © 2013-2016 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree® and XLamp® are registered trademarks and the Cree logo is a trademark of Cree, Inc. This document is provided for informational purposes only and is not a warranty or a specification. For product specifications, please see the data sheets available at www.cree.com. For warranty information, please contact Cree Sales at [email protected]. 4 Optimizing XQ & XH PCB Thermal Performance Chart 1 and Chart 2 show the relative Θj-a of various inter‑LED spacings compared to a 10‑mm spacing, the largest spacing that was simulated and where minimal cross-talk was observed. There was minimal thermal crosstalk for the LEDs mounted on an MCPCB. There was more thermal crosstalk for the LEDs mounted on an FR‑4 PCB, XH however the amount of crosstalk was small even for the smallest thermal cross-talk XQ_Thermal_AppNote_Data_v2.xlsx XQ_Thermal_AppNote_Data-mod.xlsx spacings. XQ thermal cross-talk 110% MCPCB FR4 Qj-a (°C/W) of 10-mm spacing Qj-a (°C/W) of 10-mm spacing 110% 105% 100% 95% MCPCB FR4 105% 100% 95% 90% 90% 0 2 4 6 8 Distance between LEDs (mm) 10 0 12 Chart 1: Thermal crosstalk simulation results for XQ family LEDs 2 4 6 8 Distance between LEDs (mm) 10 12 Chart 2: Thermal crosstalk simulation results for XH family LEDs Copper Trace Size We investigated the copper trace area needed below XQ and XH family LEDs to optimize thermal flow by comparing the Θj-a of a single XQ-D and XH-G LED with varying copper trace sizes. The trace size was defined as the area of the trace, kept square and centered on the board. Figure 6 and Figure 7 show images of the LEDs mounted on the various traces and the thermal simulation images. 3.3 mm X 3.3 mm 5.5 mm X 5.5 mm 7 mm X 7 mm 10 mm X 10 mm 15 mm X 15 mm Figure 6: XQ-D LED mounted on various-size traces (top) and thermal simulation of XQ-D LED mounted on same various‑size traces (bottom) Copyright © 2013-2016 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree® and XLamp® are registered trademarks and the Cree logo is a trademark of Cree, Inc. This document is provided for informational purposes only and is not a warranty or a specification. For product specifications, please see the data sheets available at www.cree.com. For warranty information, please contact Cree Sales at [email protected]. 5 Optimizing XQ & XH PCB Thermal Performance 3.3 mm X 3.3 mm 5 mm X 5 mm 10 mm X 10 mm 15 mm X 15 mm 25 mm X 25 mm Figure 7: XH-G LED mounted on various-size traces (top) and thermal simulation of XH-G LED mounted on same various-size traces (bottom) Chart 3 and Chart 4 show a comparison of the relative Θj-a of a single LED for each copper trace size on an MCPCB and an FR-4 PCB. There is a minimal difference in Θj-a for the various trace sizes on the MCPCB. There is a significant increase in Θj-a for small trace sizes and a Θj-acopper difference is due to the much better thermal conduction of the significant decrease in Θj-a for larger trace sizes on the FR-4 PCB. TheXH trace size XQ_Thermal_AppNote_Data-mod.xlsx XQ copper trace size XQ_Thermal_AppNote_Data-mod.xlsx MCPCB compared to the lesser thermal conduction of the FR-4 board. 300% MCPCB FR4 250% qj-a (°C/W) of 25-mm trace Qj-a (°C/W) of 15-mm trace 300% 200% 150% 100% 50% 0% 0 5 10 15 20 Copper trace size (mm) 25 Chart 3: Copper trace size comparison for XQ family LEDs 30 MCPCB FR4 250% 200% 150% 100% 50% 0% 0 5 10 15 20 Copper trace size (mm) 25 30 Chart 4: Copper trace size comparison for XH family LEDs Usefulness of Thermal Vias An inexpensive way to improve thermal transfer for FR-4 PCBs is to add thermal vias - typically plated through-holes (PTH) between conductive layers. Vias are created by drilling holes and copper plating them. Because the XQ and XH family LEDs do not have an electrically isolated thermal pad, it is not possible to locate thermal vias directly underneath the LEDs, so we simulated locating thermal Copyright © 2013-2016 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree® and XLamp® are registered trademarks and the Cree logo is a trademark of Cree, Inc. This document is provided for informational purposes only and is not a warranty or a specification. For product specifications, please see the data sheets available at www.cree.com. For warranty information, please contact Cree Sales at [email protected]. 6 Optimizing XQ & XH PCB Thermal Performance vias outside the trace. Figure 8 shows an XQ and XH family LED mounted on traces of three sizes with thermal vias around the outside of the trace. 3.3 mm X 3.3 mm trace 10 mm X 10 mm trace 15 mm X 15 mm trace Figure 8: Thermal vias around the trace of XQ-D (top) and XH-G (bottom) LEDs Chart 5 and Chart 6 show the Θj-a of plated and filled vias around various trace sizes compared to the Θj-a with no vias. Thermal vias can be helpful with smaller trace sizes, but are of decreasing help as trace size increases. We added a second row of traces for some of the scenarios, but the second row produced no difference in the Θj-a. Copyright © 2013-2016 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree® and XLamp® are registered trademarks and the Cree logo is a trademark of Cree, Inc. This document is provided for informational purposes only and is not a warranty or a specification. For product specifications, please see the data sheets available at www.cree.com. For warranty information, please contact Cree Sales at [email protected]. 7 Optimizing XQ & XH PCB Thermal Performance XH thermal vias XQ_Thermal_AppNote_Data-mod.xlsx XQ thermal vias 110% No Vias Plated Vias Filled Vias 105% qj-a (°C/W) of no vias Θj-a (°C/W) of no vias 110% No Vias Plated Vias Filled Vias 105% XQ_Thermal_AppNote_Data_v2.xlsx 100% 95% 90% 85% 100% 95% 90% 85% 80% 80% 0 5 10 15 Trace size (mm) 20 25 30 0 Chart 5: Thermal via results for XQ family LEDs 5 10 15 Trace size (mm) 20 25 30 Chart 6: Thermal via results for XH family LEDs FR‑4 Board Thickness Chart 7 compares the relative Θj-a of two standard thicknesses of FR-4 PCBs with various copper trace sizes to that of an 0.8‑mm thick Θj-a (°C/W) of 0.8 mm thick with 10-mm trace is recommended for thermally demanding applications. PCB with a 10 mm X 10 mm The thinner FR-4 PCB had a lower Θj-a and XQtrace. FR4 board thickness XQ_Thermal_AppNote_Data-mod.xlsx 300% 0.8 mm thick, 2 oz trace 1.6 mm thick, 2 oz trace 250% 200% 150% 100% 50% 0% 0 5 10 15 Trace size (mm) 20 25 30 Chart 7: FR-4 PCB thickness comparison FR‑4 Trace Thickness To determine how useful thicker traces are for dissipating heat on an FR-4 PCB, Chart 8 and Chart 9 compare the relative Θj-a of three thicknesses of FR-4 traces with various copper trace sizes to that of a 2‑oz copper trace. The thicker traces have better Θj-a than the thinnest trace, but increasing from 2 oz to 3 oz is a less significant improvement than increasing from 1 oz to 2 oz. Copyright © 2013-2016 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree® and XLamp® are registered trademarks and the Cree logo is a trademark of Cree, Inc. This document is provided for informational purposes only and is not a warranty or a specification. For product specifications, please see the data sheets available at www.cree.com. For warranty information, please contact Cree Sales at [email protected]. 8 Optimizing XQ & XH PCB Thermal Performance XH FR4 trace thickness XQ_Thermal_AppNote_Data-mod.xlsx 125% 125% 120% 120% qj-a (°C/W) of 2-oz copper qj-a (°C/W) of 2-oz copper XQ FR4 trace thickness 1 oz trace 2 oz trace 3 oz trace 115% 110% 105% 100% 95% XQ_Thermal_AppNote_Data-mod.xlsx 1 oz trace 2 oz trace 3 oz trace 115% 110% 105% 100% 95% 90% 90% 0 5 10 15 Trace area (mm) 20 25 0 30 Chart 8: FR-4 trace thickness comparison for XQ family LEDs 5 10 15 Trace size (mm) 20 25 30 Chart 9: FR-4 trace thickness comparison for XH family LEDs MCPCB Trace Thickness To determine how useful thicker traces are for dissipating heat on an MCPCB, Chart 10 and Chart 11 compare the relative Θj-a of three thicknesses of MCPCB traces with various copper trace sizes to that of a 2‑oz copper trace. The thicker traces have better Θj-a than the XQ MCPCB trace thickness XQ_Thermal_AppNote_Data-mod.xlsx XH MCPCB trace thickness XQ_Thermal_AppNote_Data-mod.xlsx thinnest trace but increasing from 2 oz to 3 oz is a less significant improvement. 125% 1 oz trace 2 oz trace 3 oz trace 120% qj-a (°C/W) of 2-oz trace 120% qj-a (°C/W) of 2-oz trace 125% 1 oz trace 2 oz trace 3 oz trace 115% 110% 105% 100% 115% 110% 105% 100% 95% 95% 90% 90% 0 5 10 15 Trace size (mm) 20 25 Chart 10: MCPCB trace thickness comparison for XQ family LEDs 30 0 5 10 15 Trace size (mm) 20 25 30 Chart 11: MCPCB trace thickness comparison for XH family LEDs MCPCB Dielectric Thermal Conductivity Chart 12 shows the simulated Θj-a with varying thermal conductivity of the dielectric of an MCPCB. Although here is some improvement in performance when the dielectric conductivity increases above 2 W/mK, it is not particularly significant. However there is a large decrease in thermal performance below 2 W/mK. Copyright © 2013-2016 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree® and XLamp® are registered trademarks and the Cree logo is a trademark of Cree, Inc. This document is provided for informational purposes only and is not a warranty or a specification. For product specifications, please see the data sheets available at www.cree.com. For warranty information, please contact Cree Sales at [email protected]. 9 Optimizing XQ & XH PCB Thermal Performance XQ MCPCB dielectric thermal conductivity XQ_Thermal_AppNote_Data-mod.xlsx qj-a (°C/W) of 2.2 W/mK 250% 200% 150% 100% 50% 0% 0 2 4 6 Dielectric conductivity (W/mK) 8 10 Chart 12: MCPCB dielectric thermal conductivity FR-4 vs. MCPCB 13 and Chart 14 compare the relative Θj-a of four FR-4 PCBs withXH various copper trace sizes to that of an MCPCB. For smaller traces, FR-4 vs. MCPCB XQ_Thermal_AppNote_Data_ XQ FR-4Chart vs. MCPCB XQ_Thermal_AppNote_Data_v3.xlsx there is a very large thermal conductivity penalty for using an FR-4 PCB, but the penalty decreases as the trace size increases. 400% FR4: 1 oz trace FR4: 2 oz trace FR4: 2 oz trace with vias FR4: 3 oz trace MCPCB 350% 300% 250% qj-a (°C/W) of FR-4 to MCPCB qj-a (°C/W) of FR-4 to MCPCB 400% 200% 150% 100% 50% FR4: 1 oz trace FR4: 2 oz trace FR4: 2 oz trace with vias FR4: 3 oz trace MCPCB 350% 300% 250% 200% 150% 100% 50% 0% 0% 0 5 10 15 Trace size (mm) 20 25 Chart 13: FR-4 vs. MCPCB comparison for XQ family LEDs 30 0 5 10 15 Trace size (mm) 20 25 30 Chart 14: FR-4 vs. MCPCB comparison for XH family LEDs Measured vs. Simulated Results To verify the validity of the simulations herein, we measured and simulated the Θj-a of an XLamp XQ‑D and an XH-G LED, each mounted on a 25 mm X 25 mm FR-4 PCB on a heat sink, as shown in Figure 9. We measured and simulated FR-4 PCBs of 0.8 mm thickness with various trace sizes. Copyright © 2013-2016 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree® and XLamp® are registered trademarks and the Cree logo is a trademark of Cree, Inc. This document is provided for informational purposes only and is not a warranty or a specification. For product specifications, please see the data sheets available at www.cree.com. For warranty information, please contact Cree Sales at [email protected]. 10 Optimizing XQ & XH PCB Thermal Performance Figure 9: XQ-D (left) and XH-G (right) thermal measurement configurations XQ measured - .8 mm XQ_Thermal_AppNote_Data-mod.xlsx Chartvs. 15simulated shows that the measured data very closely matches the simulation data. vs. simulated - .8 mm XH measured 120 Simulated 2 oz trace Measured 2 oz trace 100 80 qj-a (°C/W) qj-a (°C/W) 120 Simulated 2 oz trace Measured 2 oz trace 100 XQ_Thermal_AppNote_Data_v4.xlsx 60 40 80 60 40 20 20 0 0 0 5 10 15 Trace size (mm) 20 25 30 0 5 10 15 20 25 30 Trace size (mm) Chart 15: Comparison of measured and simulated Θj-a for XQ (left) and XH (right) 0.8 mm FR-4 PCB Trace Size Recommendations Cree recommends the following trace sizes for optimized heat transfer without using vias or an overly large trace. FR-4 PCB with XQ Family LED • 2 oz copper • 0.8 mm thick • 10 mm X 10 mm trace Copyright © 2013-2016 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree® and XLamp® are registered trademarks and the Cree logo is a trademark of Cree, Inc. This document is provided for informational purposes only and is not a warranty or a specification. For product specifications, please see the data sheets available at www.cree.com. For warranty information, please contact Cree Sales at [email protected]. 11 Optimizing XQ & XH PCB Thermal Performance MCPCB with XQ Family LED • 2 oz copper • 1.0 mm thick • 5 mm X 5 mm trace • thin (75 µm) dielectric with thermal conductivity ≥ 2.0 W/mK FR-4 with XH Family LED • ≥ 2 oz copper • ≤ 0.8 mm thick • ≥ 15 mm X 15 mm trace MCPCB with XH Family LED • ≥ 2 oz copper • ≤ 1.0 mm thick • ≥ 5 mm X 5 mm trace • thin (75 µm) dielectric with thermal conductivity ≥ 2.0 W/mK Chemical Compatibility As with any LED-based illumination system, it is important to verify chemical compatibility when selecting thermal interface materials, as well as other materials to which the LEDs can be exposed. Certain materials can outgas and react adversely with the materials in the LED package, especially at high temperatures when a non-vented secondary optic is used. This interaction can cause performance degradation and product failure. Consult Cree’s Chemical Compatibility application note for compounds and products safe for use with XLamp LEDs. Consult your PCB manufacturer to determine which materials it uses. Copyright © 2013-2016 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree® and XLamp® are registered trademarks and the Cree logo is a trademark of Cree, Inc. This document is provided for informational purposes only and is not a warranty or a specification. For product specifications, please see the data sheets available at www.cree.com. For warranty information, please contact Cree Sales at [email protected]. 12