LED Fundamentals

External Thermal Resistance –Resistance Interface Material and Heat SinksHeat Sinks

01-15-2012

T

External Thermal Resistance Rth BA

Th lTJunction

RthJS LED

Thermal Interface Material (TIM)

Heat sink Rth BA

TSolder Point

thJS

RthSB Substrate Technology

TBoard

RthBAHeat Sink

Ambient

thBA

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

PCBOSLON SSL

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

Thermal Interface MaterialsDescription Material Advantages Disadvantages

Thermal Grease

Typically silicone based with thermal conductive particles Thinnest joint with

minimal pressure High thermal

Flow out beyond the edges Messy for high volume

manufacturing Greased joints can pump outImprovement of thermal grease High thermal

conductivity No delamination issues

Greased joints can pump out and separate with time Compounds requires curing

process

Thermal ConductiveCompound

Improvement of thermal grease. Compounds are converted to a cured rubber film after application at the thermal interface

Phase ChangeMaterials

Typically polyester or acrylic materials with low glass transition temperature filled with conductive particles

Easy handling and installation No delamination issues No curing

Requires attachment pressure Pre-treatment heating

necessaryg

Thermal Conductive Elastomers

Typically silicone elastomer pads filled with thermally conductive particles. Often reinforced with glass fiber or dielectric films No pump out or migration

Delamination issues Thermal conductivity isdielectric films No pump out or migration

concerns No curing required

Thermal conductivity is moderate Requires attachment pressureThermal

Conductive Adhesive

Typically double sided pressure adhesive films filled with sufficient particles to balance their thermal and

LED Fundamentals | External Thermal Resistance-Interface Materials and Heat Sinks | Page 4

Tapes adhesive properties

Housing MaterialS

Housing Material

Housing

Standard Plastic = 0.3 Wm-1K-1

Thermal Enhanced Plastic = 8 Wm-1K-1

• Attachment of PCB to housing

Influencing Factors

• Attachment of PCB to housing • Housing material with high

thermal conductivity• Attachment of heat sink• Active cooling (fan)

TJ = 73°CT = TJ - Tamb = 48°C

TJ = 57°CT = TJ - Tamb = 32°C

- 16%

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

Heat Sinks

Influencing Factors

• Air flow conditions

Influencing Factors

• Air flow conditions• Material of heat sink• Orientation with respect to

gravitygravity• Base plate thickness• Cross sectional geometry• Fin geometry

Conduction

Convection and Radiation

g y• Number of fins• Spacing of fins

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

External Thermal Resistance RthBA

TS

TJReference Points:

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

OSLON SSL 1W on MCPCB in free convection

63°C 54°C 48°C

S ld P i t T t T 56°C 47°C 41°CSolder Point Temperature TS 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

LED Fundamentals | External Thermal Resistance-Interface Materials and Heat Sinks | Page 7

Heat Sink Estimation

T t l P Di i ti f th tDefine boundaryconditions

• Total Power Dissipation of the system• Max. Ambient Temperature Tamb

• Junction Temperature Tj

Estimate systemthermal resistance

total

AJth P

TTRJA

Estimate heat sinkthermal resistance JSSBJABA ththth RRR thR

Select heat sinkth l i t

Using the estimated RthBA as a target for a possible heat sink profile d i i th f i th li t lthermal resistance and examining the performance curve in the supplier catalogue

Check the design with thermal measurement when physical prototypes are available

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g p y p yp

Heat Sink

Select or design an appropriate heat sink from the RthBA calculated

LED Fundamentals | External Thermal Resistance-Interface Materials and Heat Sinks | Page 9

Verify the design with a prototype and a thermal measurement

Thank you for your attention.

LED Fundamentals | External Thermal Resistance-Interface Materials and Heat Sinks | Page 10

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 OSRAMproduct of one of OSRAM Opto Semiconductor GmbH s partners. Furthermore, OSRAM 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