External Thermal Resistance - Interface Materials and Heat Sinks: LED Fundamentals

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In this presentation we will look at thermal interface materials, influencing factors for heat sink selection, and steps to identify the right heat sink for an LED system design.

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External Thermal Resistance - Interface Materials and Heat Sinks: LED Fundamentals

  1. 1. LED FundamentalsExternal ThermalResistance –Interface Material andHeat Sinks01-15-2012
  2. 2. External Thermal Resistance Rth BA TJunction Thermal Th l Interface Material Heat sink Rth BA (TIM) RthJS LED TSolder Point RthSB Substrate Technology TBoard Heat Sink RthBA Ambient OSLON SSL PCBLED Fundamentals | External Thermal Resistance-Interface Materials and Heat Sinks | Page 2
  3. 3. Thermal Interface Materials Air Thermal Interface MaterialThe purpose of thermal interface materials is to reduce or fill the air gaps in order to improve the heat flow.LED Fundamentals | External Thermal Resistance-Interface Materials and Heat Sinks | Page 3
  4. 4. Thermal Interface Materials Description Material Advantages Disadvantages Thermal Typically silicone based with thermal  Flow out beyond the edges Grease conductive particles  Thinnest joint with  Messy for high volume minimal pressure manufacturing Improvement of thermal grease grease.  High thermal  Greased joints can pump out Thermal Compounds are converted to a cured conductivity and separate with time Conductive rubber film after application at the  No delamination issues  Compounds requires curing Compound thermal interface process  Easy handling and Phase Typically polyester or acrylic materials installation  Requires attachment pressure Change with low glass transition temperature  Pre-treatment heating  No delamination issues Materials filled with conductive particles necessary  No curing g Typically silicone elastomer pads filled Thermal with thermally conductive particles. Conductive Often reinforced with glass fiber or  Delamination issues Elastomers  No pump out or migration  Thermal conductivity is dielectric films concerns moderate Thermal Typically double sided pressure  No curing required  Requires attachment pressure Conductive adhesive films filled with sufficient Adhesive particles to balance their thermal and Tapes adhesive propertiesLED Fundamentals | External Thermal Resistance-Interface Materials and Heat Sinks | Page 4
  5. 5. Housing Housing Material Housing Material Standard Plastic S Thermal Enhanced Plastic  = 0.3 Wm-1K-1  = 8 Wm-1K-1 Influencing Factors • Attachment of PCB to housing • Housing material with high thermal conductivity • Attachment of heat sink • Active cooling (fan) - 16% TJ = 73°C TJ = 57°C T = TJ - Tamb = 48°C T = TJ - Tamb = 32°C External Thermal Resistance RthBA Characterize the heat transfer from the board to the environment.LED Fundamentals | External Thermal Resistance-Interface Materials and Heat Sinks | Page 5
  6. 6. Heat Sinks Influencing Factors Influencing Factors • Air flow conditions • Material of heat sink • Orientation with respect to gravity • Base plate thickness • Cross sectional geometry • Fin geometry g y Conduction • Number of fins Convection and Radiation • Spacing of finsLED Fundamentals | External Thermal Resistance-Interface Materials and Heat Sinks | Page 6
  7. 7. External Thermal Resistance RthBA Reference Points: TJ TS TB Tamb= 25 °C OSLON SSL 1W on MCPCB in free convection 25.4 x 25.4 x 12.30 mm³ 31.75 x 31.75 x 12.30 mm³ 50.80 x 50.80 x 17.38 mm³ Junction Temperature TJ 63°C 54°C 48°C Solder P i t T S ld Point Temperature TS t 56°C 47°C 41°C Board Temperature TB 52°C 43°C 37°C Thermal Resistance RthBA 27 K/W 18 K/W 12 K/W Thermal Resistance RthJA 38 K/W 29 K/W 23 K/WLED Fundamentals | External Thermal Resistance-Interface Materials and Heat Sinks | Page 7
  8. 8. Heat Sink Estimation • T t l Power Dissipation of th system Total P Di i ti f the t Define boundary • Max. Ambient Temperature Tamb conditions • Junction Temperature Tj TJ  TA Estimate system R th  thermal resistance JA Ptotal Estimate heat sink thermal resistance R thBA  R th JA  R th SB  R thJS Select heat sink Using the estimated RthBA as a target for a possible heat sink profile thermal resistance th l i t and examining th performance curve i th supplier catalogue d i i the f in the li t l Check the design with thermal measurement when physical prototypes are available g p y p ypLED Fundamentals | External Thermal Resistance-Interface Materials and Heat Sinks | Page 8
  9. 9. Heat Sink Select or design an appropriate heat sink from the RthBA calculated Verify the design with a prototype and a thermal measurementLED Fundamentals | External Thermal Resistance-Interface Materials and Heat Sinks | Page 9
  10. 10. Thank you for your attention.LED Fundamentals | External Thermal Resistance-Interface Materials and Heat Sinks | Page 10
  11. 11. Disclaimer All information contained in this document has been checked with the greatest care. OSRAM Opto Semiconductors GmbH can however, not be made liable for any damage that occurs in connection with the use of these contents. OSRAM Opto Semiconductor GmbH makes no representations and warranties as to a possible interference with third parties intellectual property rights in view of products originating from one of OSRAM Opto Semiconductor GmbHs partners, or in view of products being a combination of an OSRAM Opto Semiconductor GmbHs product and a product of one of OSRAM Opto Semiconductor GmbHs partners. Furthermore OSRAM GmbH s partners Furthermore, Opto Semiconductors GmbH cannot be made liable for any damage that occurs in connection with the use of a product of one of OSRAM Opto Semiconductor GmbHs partners, or with the use of a combination of an OSRAM Opto Semiconductor GmbHs product and a product of one of OSRAM Opto Semiconductor GmbHs partners.LED Fundamentals | External Thermal Resistance-Interface Materials and Heat Sinks | Page 11

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