Application Note

Light Emitting Diode

Thermal Design Considerations for the
Nichia NCSWE13x, NCSxE17x, or
NVSxE21x Series LEDs

Table of contents
1. Overview ............................................................................... 2
2. Structure and Thermal Path of the NCSWE13x, NCSxE17x,
and NVSxE21x LEDs............................................................ 2
3. Thermal Design ..................................................................... 4
4. Thermal Resistance of the LEDs ........................................... 9
5. Junction Temperature (TJ) ................................................... 12
6. Thermal Evaluation ............................................................. 15
7. Summary.............................................................................. 17

NCSWE13x, NCSxE17x, and NVSxE21x refer to Nichia part numbers. These Nichia partnumbers within this document are merely Nichia’s part numbers for those Nichia products and are
not related nor bear resemblance to any other company’s product that might bear a trademark.

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

Light Emitting Diode

1. Overview
Thermal design is very important for LEDs since it determines the optical and electricalcharacteristics (e.g. luminous flux, forward voltage, etc.) of the LEDs and it even affects the lifetimeof the LEDs. When performing a thermal evaluation, check how high the junction temperature of
the LED will be when the LED is operated in the conditions/environments in which the LED will
actually be used.
Nichia’s NCSWE13x, NCSxE17x, and NVSxE21x LEDs are much smaller in size than other LEDs
that produce the same amount of light energy and they have a high luminous flux density. Note thatthe thermal design may be challenging since the input power per unit area of the package for theLEDs is higher than for conventional LEDs.
This application note provides the thermal design considerations for these LEDs, the results of thethermal evaluations Nichia performed, and how to evaluate the junction temperature (TJ) of theLEDs in the chosen application.

2. Structure and Thermal Path of the NCSWE13x, NCSxE17x, and
NVSxE21x LEDs
For the NCSWE13x, NCSxE17x, and NVSxE21x LEDs, the light emitting device (hereinafterreferred to as “chip”) is soldered directly to an aluminum PCB to ensure that the heat that is
generated at the chip is efficiently dissipated to the atmosphere. With this structure, the heatgenerated at the chip can be transferred and dissipated efficiently as noted below and shown in
Figure 1:
LED chip → Electrode → Solder → Copper Layer → Insulation Layer → Aluminum → Heatsink

Phosphor

LED Chip

Thermal Path

Electrode
SolderCopper Layer
Insulation Layer
Aluminum PCBHeatsink

Figure 1. Reference Image of the Structure of the NCSWE13x, NCSxE17x, and
NVSxE21x LED and Thermal Path
The size of the electrodes are relatively small in proportion to the outline dimensions of the package

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Application Note
for the NCSWE13x1, NCSxE17x, and NVSxE21x LEDs; materials to be used in the thermal path
between the electrodes and heatsink for these LEDs must have sufficiently high heat dissipation. If
a PCB with a low heat dissipation is used (e.g. glass epoxy resin PCB), the heat may not beappropriately dissipated causing the junction temperature of the LED to become extremely high,the luminous flux to be reduced, etc.
See Figures 2, 3, and 4 for the dimensions of the electrodes of the NCSWE13x, NCSxE17x, and
NVSxE21x LEDs.
1.25

0.27～0.35

0.84

1.25

2- 0.84

0.2

Light Emitting Diode

CathodeAnode(Unit: mm)

Figure 2. Outline Dimensions of the NCSWE13x LED

The outline dimensions of the NCSWE13x LEDs are geometrically similar to those of the
NCSxE17x LEDs; the NCSWE13x LEDs are smaller. The NCSWE13x and NCSxE17x LEDs use
the same chip.
1.7

0.27～0.35

0.86 ①1.7

2- 0.86 ①

0.2

CathodeAnode(Unit: mm)

Figure 3. Outline Dimensions of the NCSxE17x LED

2.1

0.27～0.35

1.26 ②2.1

2- 1.26 ②

0.2

CathodeFigure 4. Outline Dimensions of the NVSxE21x LED

1

Anode

(Unit: mm)

The size of the electrodes for the NCSWE13x are not as small in proportion to the outline dimensions of the package when comparedto the packages of the NCSxE17x and NVSxE21x LEDs.
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Application Note
For the NCSxE17x and NVSxE21x LEDs, the dimensions of the electrodes are different dependingon the product version. See the applicable specifications for details.
① NCSxE17A: 0.86mm, NCSxE17A-V1: 0.84mm② NVSxE21A: 1.26mm, NVSxE21A-V1: 1.24mm

3. Thermal Design
3.1 Thermal Resistance Model and Parameters

Light Emitting Diode

Figure 5 shows a cross-sectional view of a NCSWE13x, NCSxE17x, or NVSxE21x LED mounted
on an aluminum PCB with heat dissipation using a heatsink and Figure 6 shows a simplified thermal
resistance model for these LEDs.

TT :LED Surface Temperature

NCSWE13x : 2.125mm
NCSxE17x : 2.35mm
NVSxE21x : 2.55mm
TA :Ambient Temperature

TJ :LED Junction Temperature

TB :PCB Temperature
TC :LED Case Temperature

Φ1mm

Figure 5. Cross-sectional View of the LED

TT :LED Surface Temperature
RTJ :Thermal Resistance of the Phosphor Layer

TJ :LED Junction Temperature
RθJC :Thermal Resistance of the LED
TB :PCB Temperature

TC :LED Case Temperature

RP1 :PCB Thermal Resistance 1

RP2 :PCB Thermal Resistance 2
RC :Thermal Resistance of the Heat Dissipating Material
TA :Ambient Temperature

Figure 6. Simplified Thermal Resistance Model

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Application Note
Table 1. Terms, Descriptions, and Main Precautions

Light Emitting DiodeTerm
LED Surface
Temperature

SymbolTT

LED Junction
Temperature

TJ

Thermal Resistanceof the LEDRθJC

LED Case
TemperatureTC

PCB Temperature

TB

PCB Thermal
Resistance 1

RP1

Thermal Resistanceof from the LED to
the PCBRθJB

PCB Thermal
Resistance 2

RP2

Thermal Resistanceof Thermal
Dissipation Material
Ambient
Temperature

Descriptions and Cautions/Suggestions
Temperature of the emitting surface of the LED.
The temperature of the LED chip. The maximum junction temperature of theNCSWE13x, NCSxE17x, and NVSxE21x LED is 135°C. To achieve longer
reliability, ensure that the LEDs are operated at a junction temperature of 100°C
or less.
Thermal resistance from the LED chip to the LED electrode of the LED.
The RθJC is obtained according to the measurement/calculation methods detailed
in JESD 51-1 standard. The maximum thermal resistance of the NCSWE13x andNCSxE17x LEDs is 1°C/W and for NVSxE21x LED it is 0.6°C/W, which arevery low.
Temperature of the electrode of the LED.
The TC is used in a thermal simulation; it cannot be measured once the LED issoldered to the PCB.
PCB surface temperature near the side of the LED package.
For the NCSWE13x, NCSxE17x, and NVSxE21x LEDs, since there are no solder
fillets at the electrodes and it is difficult to measure the temperature at thesoldered area directly, the measurement should be conducted at TB point shownin Figure 5.
Thermal resistance from the TC point of the LED to the TB measurement point on
the PCB.
The value of the RP1 varies depending on the soldering pad pattern for the LEDs,
the copper area and heat dissipation performance of the aluminum PCB, heatsink,
the operating temperature, etc.
Thermal resistance from the LED chip to the T B measurement point on the PCB.
This thermal resistance value can be obtained by using the following equation:RθJB = RθJC + RP1
Combined thermal resistance of an aluminum PCB in the vertical direction fromthe soldered area towards the heatsink.
The thermal resistance of an aluminum PCB varies significantly depending on theproperties of the insulating layer (thermal conductivity, thickness). Take this into
consideration when selecting materials.
The thermal resistance of an aluminum PCB can be obtained by using thefollowing equation:
Thermal resistance (°C/W) = Thickness of the insulating layer (m)/(Thermal
conductivity of the insulating layer [W/m∙K] × Its area [m²])RC

Combined thermal resistance of a thermal sheet, thermal grease, heatsink, etc.

TA

Ambient temperature of the LED assembled in the chosen application.

Section 3.2 explains the importance of what PCB is selected for the LEDs based on the results ofthe thermal simulations Nichia performed.

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Application Note
3.2 Thermal Simulations for PCBs
Since the electrodes of the NCSWE13x, NCSxE17x, and NVSxE21x LEDs are very small, the heatdissipation of the PCB used is important. When the chosen application requires a large electric
power, Nichia recommends using an aluminum PCB with a high thermal conductivity instead ofusing a glass resin substrate (FR4), a glass composite substrate (CEM3), or an aluminum PCB witha low thermal conductivity. For reference, Figures 7 and 8 show the thermal simulations when an
LED is mounted on aluminum PCBs with low and high thermal conductivity.
TJ becomes high.

Light Emitting Diode

Heat is notdissipated easily.

Figure 7. Aluminum PCB witha Low Thermal Conductivity

TJ becomes low.
Heat is dissipatedeasily.

Figure 8. Aluminum PCB witha High Thermal Conductivity

When an LED is mounted on an aluminum PCB with a low thermal conductivity, the heatdissipation becomes insufficient, resulting in a high junction temperature, whereas when an LED is
mounted on an aluminum PCB with a high thermal conductivity, the heat generated from the LED
can dissipate efficiently, resulting in a low junction temperature.
3.3 Two Types of Copper Layer Designs and Their Heat Dissipation Performance
There are two types of copper layer designs as explained below. Table 2 provides their advantages
and disadvantages.
Solder Mask Defined (SMD): The shape and location of the soldering pad pattern are defined by
the aperture of the solder resist that is applied to the surface of the PCB.
Non-Solder Mask Defined (NSMD): The shape and location of the soldering pad pattern are
defined by the dimensions of the copper layer, not by those of the solder resist.

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Application Note
Table 2. Comparison between SMD and NSMD
Soldering Pad
Pattern

SMD

NSMD

Appearance

Light Emitting Diode

Reference
Diagram■ Solder Paste■ Solder Mask
■ Copper Layer■ Insulation Layer■ PCB Base Material

Placement
Accuracy

Heat
Dissipation•

Not Good
Disadvantage: The coating
accuracy of the solder resist
effects the placement accuracy.
Good
Advantage: Good heat dissipation
performance due to a large copper
layer area.

■ Solder Paste■ Solder Mask
■ Copper Layer■ Insulation Layer■ PCB Base Material

Good
Advantage: The coating accuracy ofthe solder resist does not have as
much of an effect on the placement
accuracy.
Not Good
Disadvantage: Limited heatdissipation performance due to a small
copper layer area.

Placement Accuracy

The electrodes of the NCSWE13x, NCSxE17x, and NVSxE21x LEDs are very small and theclearance between the anode and cathode terminals is only 0.2mm. Thus, PCBs need to befabricated precisely. For SMD, since the dimensions and location of the soldering pad pattern are
defined by the solder resist, the location of the soldering pad pattern will deviate from the target
location if the solder resist is not applied to the PCB precisely; it will cause a difference between
the dimensions of the soldering pad patterns for the anode and cathode. These defects directly
cause a placement failure for the LEDs (e.g. open or short circuit, positional deviation, float,
solder ball); if SMD is used, it must be ensured that the PCB is fabricated precisely and thatsufficient evaluations for the LED placement are performed.
For NSMD, since the dimensions and location of the soldering pad pattern are defined by thecopper layer, positional deviation of the solder resist does not have a major effect on theplacement accuracy; NSMD provides better placement accuracy for the LEDs.
Note that once the NCSWE13x, NCSxE17x, and NVSxE21x LEDs are soldered to a PCB, theymust not be repaired. Refer to Nichia's application notes “Assembly Precautions for the Nichia
E13 Series LEDs” or “Assembly Precautions for the Nichia NCSxE17x or NVSxE21x LEDs” andperform sufficient evaluations before use.
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Application Note•

Heat Dissipation

To efficiently transfer the heat generated from the LEDs to the aluminum PCB, it is required tohave the copper layer around the electrodes as large as possible. With NSMD, it cannot be wider
than the electrode width. With SMD, it can be very large as long as the chosen PCB assembly design
allows; SMD provides better heat dissipation. Regarding the thickness of the copper layer, thethicker it is, the better the heat dissipation performance will be. However, Nichia recommends usinga PCB whose copper layer has a thickness of 35μm with consideration of imprecise etching thatcan occur when fabricating PCBs due to the narrow clearance (i.e. 0.2mm) between the electrodes.
3.4 Soldering Pad Patterns

Soldering Pad Pattern
Solder Resist Aperture

1.8

1.4
1.1

0.2

0.2

CathodeAnode

Cathode

1.66
1.26

1.26
0.86

0.2

1.24
0.84

Light Emitting Diode

Figure 9 shows the recommended soldering pad patterns for the NCSWE13x, NCSxE17x, and
NVSxE21x LEDs for the NSMD structure.

Anode

CathodeNCSWE13x LED

NCSxE17x LED

Anode

NVSxE21x LED(Unit: mm)
Figure 9. Recommended Soldering Pad Pattern for the NSMD Structure

For example, for the NCSxE17x LEDs, it is recommended that the solder resist is as long as 1.26mm
while the soldering pad pattern is only 0.86mm long; if the positional deviation of the solder resist
is less than 0.2mm, it will not affect the placement accuracy of the LEDs. Both SMD and NSMD
structures have advantages and disadvantages; ensure that the better method is selected for the
chosen application with careful consideration of the information provided in section 3.3.
Nichia measured the thermal resistance values (RθJB) of the LEDs. The parameters and evaluation
results are detailed in Section 4.

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Application Note
4. Thermal Resistance of the LEDs
Nichia measured the thermal resistance (RθJB) of the NCSxE17x and NVSxE21x LEDs using thefollowing parameters: the thermal conductivity rate of the insulating layer of an aluminum PCB
and the copper layer design. The RθJB was obtained in accordance with the measurement method
detailed in JESD 51 standard.
4.1 Specifications of the PCBs used for the Evaluation
Nichia used six different types of aluminum PCBs with insulating layers whose thermal
conductivity ranged from 1.8W/m∙K to 11.1W/m∙K. The specifications of the PCBs are shown in
Table 3.

Light Emitting Diode

Table 3. Specifications of the PCBs
Aluminum PCB2
Thermal Conductivity Rateof the Insulating Layer3
Thickness of the Insulting
Layer
Thickness of the Copper
Layer
Thickness of the Aluminum
Layer

Unit

NRA-ES1 NRA-8 NRA-E(3.0) NRA-E(6.5) NRA-H6 NRA-H10

W/m∙K

1.8

2.1

2.7

4.5

5.7

11.1

μm

120

120

120

120

120

120

μm

35

35

35

35

35

35

mm

1

1

1

1

1

1

4.2 Copper Layer Designs and PCB Traces•

The NCSxE17x LEDs

For the evaluation, Nichia used two types of copper layer designs (i.e. NSMD and SMD) and PCBs
with four different PCB traces: the minimum (0.5mm), the same as the electrode length (0.86mm),
the same as the length of the package (1.7mm), and the maximum (5mm).
Table 4. Evaluation Conditions for the NCSxE17x LEDs
Condition

A

B

C

D

Copper layer Design

NSMD

NSMD

SMD

SMD

PCB Trace Width

0.5mm

0.86mm

1.7mm

5mm

Not
Recommended

High-density
Applications

High-density
Applications

Non-high-density
Applications

Image of the PCB Trace

Recommended Use

2
3

The aluminum PCBs are manufactured by NIPPON RIKA INDUSTRIES CORPORATION.
The thermal conductivities of the insulating layers are the values stated in the aluminum PCB manufacture's data sheet, these are notguaranteed values.
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Application Note•

The NVSxE21x LEDs

For the evaluation, Nichia used two types of copper layer designs (i.e. NSMD and SMD) and PCBs
with four different PCB traces: the minimum (0.8mm), the same as the electrode length (1.26mm),
the same as the length of the LED package (2.1mm), and the maximum (5mm).
Table 5. Evaluation Conditions for the NVSxE21x LEDs
Condition

A

B

C

D

Copper layer Design

NSMD

NSMD

SMD

SMD

PCB Trace Width

0.8mm

1.26mm

2.1mm

5mm

Not
Recommended

High-density
Applications

High-density
Applications

Non-high-density
Applications

Recommended Use

4.3 Measurement Results for the Transient Thermal Resistance•

Figure 10 shows the evaluation results for the thermal resistance (RθJB) for the NCSxE17x
LEDs (sm27, R8000) at IF=700mA.

35.0

Thermal Resistance Value (℃/W)

Light Emitting Diode

Image of the PCB Trace

A: NSMD 0.5mm
30.0
B: NSMD 0.86mm
25.0
C: SMD 1.7mm
20.0
D: SMD 5mm

15.0
10.0
5.0

0.0
0

2

4

6

8

10

12

Thermal Conductivity Rate of the Insulating Layer of the Aluminum PCB (W/m∙K)

Figure 10. Thermal Resistance (RθJB) for the NCSxE17x LEDs

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Application Note•

Figure 11 shows the evaluation results for the thermal resistance (RθJB) for the NVSxE21x LEDs
(sm27, R8000) at IF=1400mA.

Light Emitting Diode

Thermal Resistance Value (℃/W)

35.0
A: NSMD 0.8mm

30.0
B: NSMD 1.26mm
25.0
C: SMD 2.1mm
20.0
D: SMD 5mm
15.0
10.0
5.0
0.0
0

2

4

6

8

10

12

Thermal Conductivity Rate of the Insulating Layer of the Aluminum PCB (W/m∙K)

Figure 11. Thermal Resistance (RθJB) for the NVSxE21x LEDs
For the NCSxE17x and NVSxE21x LEDs, the larger the thermal conductivity rate of the insulating
layer of the aluminum PCB was, the smaller the thermal resistance became. The thermal resistance
was smaller for the copper layer design of SMD than for NSMD. Also, the larger the width of the
PCB trace was, the smaller the thermal resistance became.
4.4 Recommended Aluminum PCB
If an aluminum PCB with a high thermal conductivity rate is used, the thermal resistance of theLEDs (RθJB) will be smaller and the effect that may be caused on the RθJB by the copper layer
design/the width of the PCB trace will be limited. For the NCSWE13x, NCSxE17x, and NVSxE21x
LEDs, Nichia recommends using an aluminum PCB whose insulating layer has a thermal
conductivity rate of ≥5.7W/m∙K.

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Application Note
5. Junction Temperature (TJ)
The junction temperature (TJ) of the LEDs is calculated using one of the following two methods:- Calculated from the PCB temperature (TB) and the input power (see section 5.1).
- Calculated by measuring the ΔVF of the LED (see section 5.2).
This section provides the detailed information on these calculation methods.
5.1 Calculating the TJ Using TB and Input PowerThe equation below is used to calculate the approximate TJ.
Equation 1: TJ = TB + RθJB  W

Light Emitting DiodeTJ = Junction Temperature (°C)
TB = PCB Temperature (°C)
RθJB=Thermal Resistance from LED
Chip to TB Measurement Point(°C/W)
W = Input Power (IFVF) (W)

Table 6. Thermal ResistancePart Number4
NCSxE17x(NCSWE13x)
NVSxE21x

SymbolUnit

PCB A5PCB B6

RθJB

℃/W

16

31

RθJB

℃/W

9

21

As explained in section 4, the RθJB will significantly vary depending on the PCB used.
Table 6 shows the RθJB of the LEDs for different PCBs; these values are obtained from theevaluation that is explained in section 4 with the severest condition for the PCB trace width (i.e.
0.5mm) and copper layer design (i.e. NSMD). The value may vary depending on the chosen
operation conditions/environments; these values should be used for reference purposes only.
•

Measurement of TB Temperature

The appropriate sensor position for a thermocouple is where it will not damage the LED when thesensor is soldered to the PCB and where it is as close as possible to the LED. Nichia recommends
placing the temperature measurement point (diameter: 1.0mm) 2.125mm from the center of the
LED for the NCSWE13x LEDs, 2.35mm for the NCSxE17x LEDs, and 2.55mm for the NVSxE21x
LEDs. Figure 12 shows a thermocouple Nichia uses. Figure 13 provides an example of how athermocouple is attached to a PCB.

Figure 12. Thermocouple (Type K)

Figure 13. Example of How a Thermocouple is
Attached to a PCB

For the NCSWE13x LEDs, refer to the RθJB for the NCSxE17x LEDs since they use the same LED chips.
Aluminum PCB whose insulating layer has a thermal conductivity rate of ≥5.7W/m∙K and a thickness of 120μm.
6 Aluminum PCB whose insulating layer has a thermal conductivity rate of ≥1.8W/m∙K and a thickness of 120μm.
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Application Note
5.2 Calculating the TJ Using the ∆VF Method
The forward voltage (VF) of an LED varies as the ambient temperature (TA) changes. The TJ
calculation method discussed in this section takes advantage of this property to calculate the T J of
the LEDs incorporated into a light module.
•

Temperature Coefficient for a Single LED on a PCB

The VF is measured with the light module placed in a constant temperature chamber and the TAchanged from 25ºC to 135ºC. The temperature coefficient is calculated from the measurement
result using the equation below.

The measuring current must be a very low current of IM=1mA; with this current, the self-heating of
the LED will not affect the VF measurement and the VF measurement will be reproducible. Figure
14 shows an example of a measurement circuit. Figure 15 shows the relationship between the TA
and VF.
Temperature Chamber
TA=25 -135°C
V[V]
VFF(V)

Light Emitting Diode

Equation 2: Temperature Coefficient, K = VF at 25°C – VF at 135°C / (135-25)

Slope = Temperature Coefficient, K

IM=1mA
[℃]
TATa
(℃)

Figure 14. Example of a Measurement Circuit•

Figure 15. Relationship between the TA and VF

TJ Calculation after Thermal Saturation

The forward voltage is measured immediately after the LED is energized (VF1) and when thesaturation temperature at the junction has been reached after aging (VF2) to calculate the ΔVF (ΔVF
＝VF1-VF2). The measuring current for the VF1 and VF2 must be a very low current of IM=1mA; withthis current, the self-heating of the LED will not affect the VF measurement and the VF measurement
will be reproducible. The aging current (IF, the current applied to have the saturation temperaturereached) must be the current that will be actually used for the chosen application. The equation
below shows the relationship between the TJ and ΔVF; the approximate TJ is obtained with thisequation.
Equation 3: TJ=∆VF/K+TA
Figure 16 shows an example of a measurement circuit. Figures 17 and 18 show an example of input
currents and VF measurement respectively for reference purposes.

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Application Note
IM=1mA (for VF1/VF2 Measurement)
IF= Aging Current

Aging
VF (V)IF (mA)[mA]
IF

Light Emitting Diode

Figure 16. Example of a Measurement Circuit

IF→
IM→

VF1→
[sec]
(sec)

Figure 17. Example of Input Currents

VF2→
(sec)

Figure 18. Example of VF Measurement

The ΔVF method provides a higher measurement accuracy than the method detailed in section 5.1;
the TJ can be calculated more accurately. However, the ΔVF method is not easy to use since it
requires equipment (e.g. constant temperature chamber, power supply to apply a measuring current,
measurement instruments, etc.) and that the measurement for the temperature coefficient K and ΔVF
must be accurate. Nichia provides a simpler temperature evaluation method in section 6.

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Application Note
6. Thermal Evaluation
Infrared thermography is an effective method to evaluate the temperatures of an LED light module.
The LED junction temperature cannot be measured by infrared thermography; however, the LED
surface temperature and the temperature distribution over the entire surface of the PCB can beobtained by infrared thermography.
Figure 19 shows an example of Nichia's thermal evaluation.
6.1 Example of Thermal Evaluation for an LED Light Module
Evaluated Module:
Measurements

Light Emitting Diode

Ar17

Ar28

PCB Outline Dimensions: 40×40mm
Light Emitting Surface (LES): φ17.4mm
P/N of the LEDs: NCSxE17x (sm27, R8000)
Number of LEDs: 48 LEDs
IF: 350mA
Input Power: 48W
Insulating Layer of the Aluminum PCB:
Thermal Conductivity: 5.7W/m∙K
Thickness: 120μm
Copper Layer Design: NSMD
Thermal Resistance of the Heatsink: 0.5°C/W

Ar39

℃

Max

84.1

Min

61.0

Average

77.6

Max

80.3

Min

58.6

Average

73.8

Max

43.0

Min

42.3

Average42.7

Parameters
Emissivity

0.95

Refl. temp

25℃

Figure 19. Thermal Evaluation Using a Thermal Imaging Camera10
6.2 LED Surface Temperature
Since the NCSWE13x, NCSxE17x, and NVSxE21x LEDs have a phosphor layer above the LED
chip, the LED surface temperature tends to be higher than the LED junction temperature. Most of
the heat is generated in the LED junction area; however, heat is also generated in the phosphor layer
due to the Stokes’ loss in the wavelength conversion. The phosphor layer of these LEDs is very thinand is located far from the heat dissipation path of the LED chip, leading to the poor heat dissipation.
Refer to Figure 20 for a schematic diagram of how the heat generated flows within these LEDs.

Ar1: Surface temperature of an LED in the middle of the LED cluster.
Ar2: Surface temperature of an LED at the edge of the LED cluster.
9 Ar3: PCB temperature.
10 Thermal Imaging Camera Used: T620 (640×480 pixel), manufactured by Teledyne FLIR LLC
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8

Application Note

Heat Generation in the Phosphor Layer: Even
though the heat is conducted via the LED chip,
the heat cannot dissipate effectively.

LED Surface Temperature
Heat Generation Source:
Phosphor Layer

ThermalConduction

ThermalConduction

LED Junction Temperature

Figure 20. Heat Generation in the Phosphor Layer

The amount of the heat generated in the phosphor layer changes depending on the color temperature,color rendering index, and light output of the LEDs. Figures 21 and 22 show the temperature
difference between the LED surface and the LED junction at different forward currents.
70

70

sm50 R70

60

sm27 R8000

50
40
30
20

10

Difference in Surface and(ºC)
Temperatures
Junction
Difference(℃)
Temperature

Difference in Surface andTemperatures (ºC)
Junction
Temperature Difference(℃）

Light Emitting Diode

Heat Generation Source:
LED Junction

sm50 R70

sm27 R8000

60
50
40
30

20
10

0

0

0

200

400
600
Operating Current(mA)

800

Forward Current (mA)

Figure 21. Relationship between the LED Surface
and Junction Temperature Differences for the
NCSxE17x LED

0

500

1000

1500

Operatng
Current(mA)
ForwardCurrent(mA)

Figure 22. Relationship between the LED Surface
and Junction Temperature Differences for the
NVSxE21x LED

For example, it is indicated in Figure 21 that when the NCSxE17x LEDs (R8000, sm27) are
operated at IF=350mA, the LED surface temperature becomes higher than the junction temperature
by approximately 30℃. Using this evaluation result, the approximate junction temperature can becalculated. See the equation below for the calculation.
Calculation for the NCSxE17x LEDs (R8000, sm27) at IF=350mA:
Approx. TJ =LED surface temperature (measured by a thermal imaging camera) - 30℃ (based on Figure 21)

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Application Note

Light Emitting Diode6.3 Precautions for Thermography Measurement•

Infrared thermography measurements will contain errors due to the measurement equipment,
measurement conditions, measurement environment, etc. When using measurement values
obtained through infrared thermography for the thermal design, the chosen application must be
designed with sufficient margins to ensure the heat is dissipated properly.

•

The LED surface temperature becomes even higher than the LED junction temperature during
operation. When using optical components (e.g. lens or reflector) with a low heat resistancetemperature, ensure that the appropriate clearance is provided between the optical component
and the LED and that the heat resistance temperature of the optical component used must not
be exceeded.

•

Nichia recommends that the LED surface temperature is 150ºC or less11. When the LED surface
temperature is high, it indicates that the LED has been operated under significant severe
conditions; ensure that the PCB used, the shape of the soldering pad pattern, the performanceof the heatsink, and the value of the forward current are appropriate for the chosen application.

7. Summary
This application note explains important information and precautions for the thermal design of theNCSWE13x, NCSxE17x, and NVSxE21x LEDs. Select suitable PCB materials and thermal
management materials for the LED light module when designing the chosen application.
Perform sufficient thermal evaluations prior to use with the chosen application with considerationof conditions/environments in which the LEDs will actually be used to ensure there are no issues.

11

The LED surface temperature should be ≤150℃ for the R70, R8000, R9050, or R9080 CRI Rank LEDs, and ≤135℃ for the Rs060
CRI Rank LED.
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Application Note

Light Emitting Diode

Disclaimer
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/or
applications 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 any
damages 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
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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

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