Application Note

Heat Dissipation Performance According to theLight Emitting DiodeAdhesion Strength of the COB and Housing

Table of contents
1. Overview ........................................................................................... 2
2. Relation of the Tightening Torque and the Heat
Dissipation ....................................................................................... 2
3. Relation between the Amount of Heat Dissipating
Grease and the Heat Dissipation ............................................. 3
4. How the Thermal Film Thickness Affects the Heat
Dissipation ....................................................................................... 5
5. Summary .......................................................................................... 6This document contains tentative information, Nichia may change the contents without notice.
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Application Note
1. Overview

Light Emitting DiodeCOBs have been recently introduced in LED lighting applications and expanded into themarket. It is very necessary to consider heat dissipation to design the lighting fixture
installing COBs.
A single COB can achieve a high output power. Moreover, the design of COBs allows theheat to be concentrated in a narrow space, because the multiple dice are closely mounted.
Thus, it is necessary to verify the heat dissipation when designing the lighting fixture, notto degrade the performance by heat (e.g. decrease in the luminous flux according to thetemperatures) or not to deteriorate the reliability performance.
This document shows the evaluation results of the heat dissipation performance according
to the adhesion strength of COBs and the housing (heat sink).

2. Relation of the Tightening Torque and the Heat Dissipation
Nichia recommends that the specific holder should be used to fix a COB to the housing(e.g. heat sink). Enhancing the adhesion strength of COBs to the housing can result inbetter heat dissipation performance. In general, screws are used to fix the holder; thus, thetightening torque significantly affects the adhesion strength.
We evaluated the heat dissipation performance according to the tightening torque.
2.1 Evaluation Method
A COB attached to the heat sink with holders. We changed the tightening torque for thescrews to evaluate the relation of the torque and TJ (cf. Table 1). We used Part No.
NSCWL036A (5.8W) and Part No. NSCWJ216A (34.8W) for the evaluation.
Table 1. Evaluation Method
COB

Part No. NSCWL036A

Part No. NSCWJ216A

Power

5.8W (Rated Power)

34.8W (Rated Power)

Fixed with different tightening torqueFixed with different tightening torqueOverview

HolderMade by Tyco Electronics CorporationScrew

M2.6 (Pan Head Screw, no washer)

Heat Sink
Heat Dissipating Seat

H17mm×W98mm×L100mm

H60mm×W110mm×L100mm(Thermal Resistance:3.0°C/W)(Thermal Resistance:1.3°C/W)

t=0.25mm, Thermal Conductivity:1.0W/mK

After attaching the COBs to the heat sink as seen in Table 1, each COB was operated at therated power. We measured the TJ after 30 minutes when the heat was saturated, andThis document contains tentative information, Nichia may change the contents without notice.
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Application Notecompared the TJ according to the tightening torque.
2.2 Evaluation Result
Figure 1 shows the evaluation results.
160

Part No. NSCWL036A(5.8W)

TJ Max: 130°C

140Part No. NSCWJ216A (34.8W)

120

TJ[°C]

1008060

Light Emitting Diode40

Just placed on the heat sink without holder2000.0

0.1

0.2

0.3

0.4 0.5 0.6 0.7 0.8
Tightening Torque[N・m]

0.9

1.0

Figure 1. Relation of the Tightening Torque and TJ
Based on the measurement results above, it is determined that increasing the tightening
torque, i.e. increasing the adhesion of the COBs to the heat sink, significantly affects theheat dissipation performance. To achieve better heat dissipation performance, it isnecessary to control the tightening torque during the holder attaching process.

3. Relation between the Amount of Heat Dissipating Grease andthe Heat Dissipation
Heat conductive materials are often used between the COBs and the housing (heat sink)to ensure uniform heat dissipation. When the heat dissipating grease is used as the heat
conductive material, the adhesion of the grease to the housing depends on the amount of
the grease, affecting the heat dissipation performance.
We evaluated the heat dissipation performance according to the amount of the grease.
3.1 Evaluation Method
We prepared the three kinds of samples with different amount of the grease (cf. Table 2)
to evaluate the relation between the amount of the grease and the TJ.
Table 2. Samples with Different Amount of Grease
Grease: 30%

Grease: 50%

Grease: 100%Sample

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Application Note
We prepared the samples under the following conditions:
COB

Part No. NSCWJ216A (34.8W: Rated Power)

HolderMade by Tyco Electronics Corporation

Screw

M2.6 (Pan Head Screw, no washer), Tightening Torque: 0.9N･mHeat Sink

H60mm×W110mm×L100mm (Thermal Resistance:1.3°C/W)

Heat Dissipating Grease

Thermal Conductivity:1.5W/mK250
No Grease200150

TJ [°C]Light Emitting Diode

3.2 Evaluation Result
After attaching the COBs to the heat sink, each COB was operated at the rated power. We
measured the TJ after 30 minutes when the heat was saturated, and compared the TJ
according to the amount of the grease. Figure 2 shows the evaluation results.

TJ Max: 130°C

100
5000102030

4050

60

7080

90

100

Amount of Grease (Area Ratio of Grease to the Total Application Area)

Figure 2. Relation of the Amount of the Grease and TJ
Based on the measurement results above, it is determined that, as the amount of thegrease is increased up to 100%, the heat dissipation performance is enhanced. To achieve
better heat dissipation performance, it is necessary to ensure that the reverse surface of
the COB is in full contact with the heat sink by confirming the spreading behavior according
to the amount of the grease in advance.

This document contains tentative information, Nichia may change the contents without notice.
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Application Note
4. How the Thermal Film Thickness Affects the Heat Dissipation

4.1 Evaluation Method/Conditions
LED Part Numbers: Part No. NFCWL060B-V1
Evaluation Conditions: The LEDs were attached to heat sinks. Thermal grease (forcomparison purposes) and thermal films of three different thicknesses (i.e. 0.5mm, 1.0mm
and 1.5mm) were used at the interface between the COB LED and heat sink.
COBCOB
ThermalThermalfilm
film

Housing/HeatSink
Housing/Heat Sink

4.2 Evaluation Results
Refer to Figures 3 and 4 below for details.
32.52
θJC [°C/W]
RθJCR[℃/W]

Light Emitting DiodeIf a thermal film is used for the interface between the COB LED and housing/heat sink todissipate the heat effectively, thermal films with low thermal resistance should be chosen.
The thermal resistance of thermal films varies depending on the film thickness. If the film
thickness is excessive, the thermal resistance between the die junction and TC
measurement point (i.e. RθJC) may be larger than the RθJC indicated in the specification.
Refer to the following sections for further details.

1.5

10.5
0Thermal Grease Thermal film
(Reference)
t=0.5mm

Thermal film
t=1.0mm

Thermal film
t=1.5mmFigure 3. Thermal Film Thickness vs. RθJCThis document contains tentative information, Nichia may change the contents without notice.
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Application Note
TJ calculated using the RθJC indicatedin the specification
Measured TJ [°C]125

TC /TTCJ /[°C]
TJ [℃]

100

Measured TC [°C]755025Light Emitting Diode

Thermal Grease Thermal film
(Reference)
t=0.5mm

Thermal film
t=1.0mm

Thermal film
t=1.5mmFigure 4. Thermal Film Thickness vs. TC/TJ
As shown in Figures 3 and 4 above, the evaluation results confirm that:- The thicker the thermal film was, the higher the RθJC.
- When the thermal film thickness was excessive, the actual TJ became larger than the TJ
calculated using the RθJC indicated in the specification.
Since the actual RθJC may vary depending on the thickness of the thermal film and thisuncertainty factor is not included in the calculation formula indicated in the specification,
another method to determine the TJ should be used at the same time when a thermal film
is used for the chosen application: A non-contact temperature measuring instrument (e.g.
thermal imaging camera) should be used to review the temperature distribution.
There is another uncertainty for thermal films (i.e. film compressibility). The thermal
resistance of thermal films varies depending on the film compressibility: The more thethermal film between the COB LED and heat sink is compressed, the lower the thermal
resistance will be. If the tightening torque used to attach the COB LED holder to a heat sinkwith screws varies, it may cause the film’s thermal resistance to change and the RθJC tobecome larger than the RθJC indicated in the specification. To prevent this from occurring,
regular monitoring/control of the tightening torque should be performed.

5. Summary
The adhesion strength between the COB LED and housing/heat sink has a strong
correlation with the heat dissipation. To improve the product quality for the chosen
application, using the evaluation results provided in this application note as a reference,
ensure that thermal simulations are performed to verify the chosen thermal design issufficient.
Note that the data/values provided in this application note may vary depending on theCOB LED model and/or the operating conditions/environment where the COB LED is used;the content of this application note should be used for reference purposes only.
This document contains tentative information, Nichia may change the contents without notice.
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Application Note

Light Emitting DiodeDisclaimer
This application note is a controlled document of Nichia Corporation (Nichia) published toprovide technical information/data for reference purposes only. By using this applicationnote, the user agrees to the following: This application note has been prepared solely for reference on the subject matters
incorporated within it and Nichia makes no guarantee that customers will see the same
results for their chosen application.
 The information/data contained herein are only typical examples of performances and/orapplications for the product. Nichia does not provide any guarantees or grant any licenseunder or immunity from any intellectual property rights or other rights held by Nichia orthird parties.
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TOKUSHIMA 774-8601, JAPAN
Phone: +81-884-22-2311 Fax: +81-884-21-0148

NICHIA CORPORATION
http://www.nichia.co.jp

This document contains tentative information, Nichia may change the contents without notice.
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