Application Note

Light Emitting Diode

Thermal Design of the LEDs

Table of contents
1. Overview ........................................................... 2
2. Path of the heat .................................................. 2
3. Calculating TJ .................................................... 3
4. Thermal Design ................................................. 4
5. Summary ............................................................ 8This document contains tentative information, Nichia may change the contents without notice.
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Application Note
1. Overview
When designing a product using LEDs, it is important to account for the heat generated by theLEDs. The ambient temperature that the LED can be used in is determined by the junction
temperature (TJ). The LEDs may become dim, or not even illuminate if the TJ goes above theabsolute maximum ratings specified in the specification sheet. It is important to have a good thermal
design as keeping the TJ as low as possible will lengthen the life of the LED.
This application note will contain information on how to have a good thermal design.

2. Path of the heat

Light Emitting DiodeFigure 1 is an example of how the heat can pass through the LED. It is believed that the heat istransmitted to the air by going through the die bond, electrodes, solder, and the board.
Bonding wire

Encapsulating resin

Chip

Package

ElectrodeSolder

Die Bonding

Board

Figure 1. LED’s structure and an example of the heat path (Example Part No. NS3W183)

Figure 2 indicates a flow chart of the heat from the chip.

Chip

Die Bonding

TJ (℃)

Electrode

SolderBoard

TS (℃)

TA (℃)RθJS
RθJA

Figure 2. Flow chart of the heat from the chipThis document contains tentative information, Nichia may change the contents without notice.
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Application Note
If the change in temperature from the chip is expressed as the thermal resistance (℃/W), twoequations can be written with regards to TJ.
(1) If using the value of the thermal resistance (RθJA) from the chip to Ambient temperature:
TJ = TA + RθJA × W ・・・・・・①
・TA: Ambient temperature(℃)
・RθJA: Thermal Resistance from Chip to ambient (℃/W)
・W: Input power consumption(=IF×VF)(W)(IF: Forward current, VF: Forward voltage)(2) If using the value of the thermal resistance (RθJS) from the chip to Soldering temperature:

Light Emitting DiodeTJ = TS + RθJS × W ・・・・・・②
・TS: Soldering temperature (Cathode side) (℃)
・RθJS: Thermal Resistance from the chip to TS measuring point (℃/W)
・W: Input power consumption(=IF×VF) (W)(IF: Forward current, VF: Forward voltage)

3. Calculating TJ
There are two methods to calculate TJ
・Calculating TJ by the TS measurement
・Calculating TJ by the VF measurement method
Details are shown below:
3.1 Calculating TJ by measuring TS(1) Attach a thermocouple to the TS measurement point (cathode side) of the mounted LED.
Turn on the LED and measure the temperature Ts, IF and VF when the LED has reached the thermal
equilibrium state.
※The TS measurement point and RθJS may vary according to the products. Please confirm it inour specifications.
※Please use the thinnest thermocouple possible to minimize the measurement interference.
The connection of the thermocouple should be done by soldering.
TS Point
Thermometer

Constant Current Source
Thermo Couple

Figure 3. Thermal Measuring Set Up
This document contains tentative information, Nichia may change the contents without notice.
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Application Note

* 本特性は参考です。
All characteristics shown are for reference only and are not guaranteed.

管理番号 No.

NS3W183
STS-DA7-0685(2) The T順電圧-順電流特性
measured values of TS, IF and VF.
J can be calculated using equation ② with the周囲温度-順電圧特性
Forward Voltage vsAmbient Temperature vs
Forward Voltage

VfIf

Forward TCurrent
3.2 Calculating
J by measuring VF

TA=25°CmA
IFP=350=350mA
Part No. NS3W183 IFP

1000

4.5

(1) The LED’s forward voltage (VF) is measured
600
at a specific increment of ambient temperature (TA)350
using a thermostatic chamber.
※The measurement is taken in a windlessenvironment. 100※Pulse current is used when measuring the LEDs
to keep the VF stable from being affected by its
own heat generation. (Recommendation:
Pulse width <10ms, Duty ratio < 1/10)

順電圧
Forward Voltage(V)

順電流
Forward Current(mA)

4.0

3.5

3.0

2.5

10

2.0

2.0

2.5

3.0

3.5

4.0

4.5

-60 -40 -20020 40 60 80 100 120(2) Making a graph of Ambient Temperature vs
順電圧
周囲温度
Forward Voltage
Forward Voltage(V)Ambient Temperature(°C)
from themeasurement result (1).
NS3W183* 本特性は参考です。
管理番号 No. STS-DA7-0685
All characteristics shown are for reference only and are not guaranteed.
Figure 4. Ambient Temperature(※We can consider TA≒TJ in a thermal equilibrium state.)
周囲温度-相対光束特性
順電流-相対光束特性
vs. Forward※Example:Figure
4 indicatesthe ambient temperature vs
ForwardCurrentvs
Ambient Temperature
vs Voltage
順電圧-順電流特性
周囲温度-順電圧特性
TaIv
IfIv
Relative Luminous Flux
Relative Luminous Flux
I =350 mA
T=25°Cforwardvoltage
properties
forVfIf
Part No. NS3W183. Ambient Temperature vs TaVf
ForwardVoltagevsFPA

Forward2.5Current

Forward1.4
Voltage

TA=25°C

1000

1.5(4) The TJ can be calculated by the VF measured,100
1.0
using the ambient temperature vs. forward voltage
graph.
0.5[Example]:
At VF=3.41(V), 0.0
the TJ is 50℃ as show in Figure 5.
100

100

2.0

2.5

200300400500

3.0 順電流3.54.0
Forward Current(mA)
順電圧
Forward Voltage(V)

600

1.2
4.0

1.0
3.5

3.0
0.8

2.5

0.6700

2.0

4.5-60 -40 -20020406080

100 12002040 60 80
周囲温度
Ambient Temperature(°C)
周囲温度
Ambient Temperature(°C)

100 120-60 -40 -20

VF=3.41VTA≒TJ=50℃

Figure 5. Calculation of TJ

4. Thermal
Design順電流-相対光束特性
Forward Current vs=350 mA
Part No. NS3W183 IFPIFP=350mA

4.5

相対光束
順電圧
ForwardLuminous Flux(a.u.)
Relative Voltage(V)

順電流
相対光束
Luminous Flux(a.u.)
RelativeCurrent(mA)
Forward(3) In a windless environment, turn on the LED with600
DC and measure2.0
the VF once it is in a thermal
350
equilibrium state.

周囲温度-相対光束特性Ambient Temperature vsIfIv

TaIv

Relative Luminous
Fluxthe TJ can be lowered by implementing
Relative Luminous
FluxmA
When designinga product,
methods
to lower theI =350
thermalT =25°C
1.4
resistance of the2.5product as a whole.
FP

Example:2.0
・Choice of board materials
・Optimization of the copper foil area of the board
1.5
・Optimization of the LED placement (LED pitch)・Implementing
heat sinks
1.0
Details of each are explained below.

相対光束
Relative Luminous Flux(a.u.)A

相対光束
Relative Luminous Flux(a.u.)Light Emitting DiodeTaVf1.2

1.00.80.5This document contains tentative information, Nichia may change the contents without notice.
0.6
0.0
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0100200300
400
500
600
700
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順電流
Forward Current(mA)

周囲温度
Ambient Temperature(°C)

Application Note
4.1 Choice of board materials
Printed circuit boards can be classified as resin, metal-based, or ceramic as shown in Figure 6.
Paper Phenol base material(FR-1, 2)

Rigid PCB

Paper Epoxy base material(FR-3)

PCB based on resin materialWoven fiber glass base material(FR-4, 5)Flexible PCB

Glassmat Polyester(FR-6)

Metallic base PCB

Composite PCB(CEM-1, 3)

Light Emitting DiodePCB based on metallic material
Metallic core PCB

PCB based on ceramic material

Alumina PCB

Figure 6. Classification of Printed Circuit Boards
Woven fiber based material (FR-4) boards are commonly used due to their cost and their dimension
consistency. However, by using a metal-based board a higher thermal conductivity can be achieved
and will lower the TJ of the LED. As a reference, Table 1 and Figure 7 show the thermal
measurement results with a FR-4 and with an aluminum board.
Table 1. Thermal Measurement Results for Part No. NS6W183
Type A

Type B

Type CType D

Appearance

Board Material
RθJA (℃/W)

FR-463PWB Size

Aluminum

50

44

34

30mm×30mm,30mm×30mm,t=1.6mm

t=1.7mm

Copper Area Face

154mm2, t=0.07mm

302mm2, t=0.07mm

616mm2, t=0.07mm

500mm2, t=0.07mm

Copper Area Back

154mm2, t=0.07mm

302mm2, t=0.07mm

616mm2, t=0.07mm-IF (mA)

700

VF(V)

3.18

3.24

3.29

3.3TS(℃）

1431189580TJ(℃）

165141118103※Measurement condition: RθJS=10℃/W, TA =25℃, Thermo Couple: Φ0.076mm
This document contains tentative information, Nichia may change the contents without notice.
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Application Note
Measurement Results Copper Area vs. TJ
200

FR-4

Type-A

Aluminium

Type-B150

TJ(℃）

Type-C
Type-D

100500Light Emitting Diode0100

200

300
400
Copper Area(mm2)500600700

Figure 7. Thermal Measurement Results for Part No. NS6W183 (Copper area vs TJ)
Assuming that the board sizes are the same, it is clear from the measurements above that the TJ islower on the aluminum board compared to the FR-4 board.
4.2 Optimization of the copper foil area of the board
To transfer the heat generated by the chip to the board as much as possible, it is recommended toincrease the thermal conductive area by increasing the area of the copper foil as shown in Figure 8.
Table 1 and Figure 7: The value of the TJ falls as the copper foil area increases.
LEDLED

Copper Foil
Foil
CopperLEDLED

Copper Foil
Foil
Copper: Solder Resist：SolderResist
Copper foil area is small.

Copper foil area is large.

(Thermal conduction is poor.）(Thermal conduction is good.)

Figure 8. Copper foil pattern shape of the printed circuit board
This document contains tentative information, Nichia may change the contents without notice.
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Application Note
4.3 Optimization of the LED placement (LED pitch)

Light Emitting DiodeIf the LED pitch becomes too narrow as shown in Figure 9, it becomes harder to radiate the heatgenerated in a concentrated area.
Figure 10 shows the heat distribution simulation with various LED pitches.
It can be seen that the heat is trapped when the LED pitch is narrow. By increasing the LED pitch
as much as possible, the TJ could be lowered.

LED Pitch: Narrow

LED Pitch: WideFigure 9. LED Placement
TS = 67.3℃

TS = 55.7℃

TS = 46.3℃

LED Pitch

LED Pitch

LED Pitch

12mm

15mm

20mm

Figure 10. Thermal distribution simulation results (Example: Part No. NSSW157)※Since the simulation is set to have the copper foil area as large as possible to in order to have thewidest LED pitch, the copper foil area will be different for each simulation.
4.4 Implementing a heat sink
The heat radiation efficiency can be improved by attaching a heat sink to the back side of the board.
Table 3 shows the measurement results with or without a heat sink. Both RθJA and TJ are lower whena heat sink is attached.
This document contains tentative information, Nichia may change the contents without notice.
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Application Note
To further improve the heat radiation efficiency, it is recommended to attach the heat sink to theboard using a thermally conductive tape, sheet, or grease.
Examples of connecting a heat sink to the board are shown in Figure 11.
Table 3. Thermal measurement results with the heat sink for Part No. NS6W183
without Heat Sink

with Heat Sink

Appearance

Board Material

Light Emitting Diode

RθJA [℃/W]

FR-4
44

PWB Size

Thermal Grease32

30mm×30mm,t=1.6mm

Copper Area

616mm2, t=0.07mm

IF (mA)

700

VF(V)

3.29

3.49

TS(℃）

9573

TJ(℃）

118

97

COB

Heat Sink
LED Board
基板
両面テープ
AdhesiveTape

ヒートシンクHeat Sink

Connection with COB and the heat sink

Connection with an LED board and the heat sink

Figure 11. Connection with the board and the heat sink

5. Summary
By following the thermal designs mentioned in this application note, LEDs can be more efficiently
used and thereby potentially improve the reliability of the end product.

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 to provide
technical information/data for reference purposes only. By using this application note, 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 license underor immunity from any intellectual property rights or other rights held by Nichia or third parties.
 Nichia makes no representation or warranty, express or implied, as to the accuracy, completenessor usefulness of any information contained herein. In addition, Nichia shall not be liable for anydamages or losses arising out of exploiting, using, or downloading or otherwise this document, or
any other acts associated with this document.
 The content of this application note may be changed without any prior or subsequent notice.
 Copyrights and all other rights regarding the content of this document are reserved by Nichia or
the right holders who have permitted Nichia to use the content. Without prior written consent of
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NICHIA CORPORATION
http://www.nichia.co.jp

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