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.

1. LED Fundamentals
External Thermal
Resistance –
Interface Material and
Heat Sinks
01-15-2012

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 PCB
LED Fundamentals | External Thermal Resistance-Interface Materials and Heat Sinks | Page 2

3. Thermal Interface Materials
Air Thermal Interface Material
The 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. 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 properties
LED Fundamentals | External Thermal Resistance-Interface Materials and Heat Sinks | Page 4

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. 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 fins
LED Fundamentals | External Thermal Resistance-Interface Materials and Heat Sinks | Page 6

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/W
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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 yp
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9. Heat Sink
Select or design an appropriate heat sink from the RthBA calculated
Verify the design with a prototype and a thermal measurement
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10. Thank you for your attention.
LED Fundamentals | External Thermal Resistance-Interface Materials and Heat Sinks | Page 10

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 GmbH's partners, or in view of
products being a combination of an OSRAM Opto Semiconductor GmbH's product and a
product of one of OSRAM Opto Semiconductor GmbH's 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 GmbH's
partners, or with the use of a combination of an OSRAM Opto Semiconductor GmbH's
product and a product of one of OSRAM Opto Semiconductor GmbH's partners.
LED Fundamentals | External Thermal Resistance-Interface Materials and Heat Sinks | Page 11