Application Note

Thermal Design of a Through Hole Substrate

Light Emitting Diodefor the Heat Dissipation of the LEDs

Table of contents
1. Objective .................................................................... 2
2. Thermal Path in an LED ........................................ 2
3. Heat Transfer to Board.......................................... 2
4. Design of Through Holes..................................... 3
5. Conclusion ................................................................ 7

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Application Note
1. Objective

Light Emitting DiodeTo design LED products, it is essential to take thermal expansion into account.
The operating temperature of the LEDs is determined by the junction temperature (TJ).
When the junction temperature exceeds the maximum value specified for each model, theluminous flux is significantly decreased, resulting in catastrophic failures such as no lightemission due to wire breakage. Therefore, it is necessary to use LED products so that thejunction temperature (TJ) does not exceed the maximum value.
The optimal TJ will enable LED products to have a long life.
It is required, therefore, to achieve the optimal thermal design for LED products.
In this application note, Nichia will recommend the design of Through Holes as a methodof heat dissipation by verifying prototypes.

2. Thermal Path in an LED
Figure 1 shows the thermal path in an LED. The heat generated from the chip is transferredthrough the die bonding resin, electrode, solder, board, and finally to the externalenvironment.
Bonding wire

Encapsulating resin

Chip

Electrode(Cathode)

Package

SolderBoard
Die Bonding

Figure 1. Cross Section of LED and Thermal Path

3. Heat Transfer to BoardTable 1 shows the simulation of heat dissipation from the chip to the board. The heatgenerated from the chip is transferred to the board in a concentric circle pattern.
Table 1. Simulation of Heat Dissipation
Board

Simulation of Heat Dissipation

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Application Note
4. Design of Through Holes

4.1 Setting of Diameter of a Through HoleWe evaluated the heat dissipation by changing the diameter of a Through Hole. (We usedthe same conditions in the number of Through Holes and the diameter of a set of ThroughHoles.)
Table 2. Verification Item: Diameter of a Through Hole
Reference Board

Diameter of a Through Hole[φd]The heat dissipation in each condition is evaluated by the ratio of TJ, regarding the TJ of
the reference board without any Through Hole as 1. Please refer to Figure 2 for theevaluation results of the heat dissipation depending on the diameter of a Through Hole.
Reference Board
1

Evaluation of Heat Dissipation
TJ vs. Diameter of a Through Hole

0.6W
0.9

Relative TJ (a.u.)Light Emitting DiodeFor the optimal design of Through Holes, we experimented with prototypes by changing
the verifying items below:Verification Item
1) Diameter of a Through Hole2) The number of Through Holes
3) The diameter of a set of Through Holes
Note: We used a two-layer FR4 (Layer thickness: 1.6 mm, Copper plating thickness: 35 µm)
for the prototypes. On the basis of the simulation of heat dissipation described inSection 3, we located the Through Holes in a circle around the LED prototype.

0.8

0.7
0.6

0.5
00.5
11.52
Diameter of a Through-hole : φd (mm)2.5Figure 2. Evaluation Result of Heat Dissipation Depending on Diameter of a Through HoleThis document contains tentative information, Nichia may change the contents without notice.
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Application Note
The measurement results in Figure 2 show that larger diameters of a Through Hole don’tnecessarily result in better heat dissipation; the optimal diameter can be read from thegraph. This is probably due to the surface area of the copper plating.
In theory, the largest surface area of copper plating ensures the best heat dissipation, whenthe maximum value is given by the formula below:(The sum of the inner surface areas of Through Holes) - (The sum of the lost top andbottom surface areas of Through Holes)

Light Emitting DiodeThe value obtained from the formula is the net increase of the surface area of copper
plating.
The net increase of the surface area of copper plating is defined by the following formula:y= (xπ・A・t)－｛(x/2)2・A・2｝= -1/2Aπ｛(x – t)2 – t2｝
Here, x: Diameter of a Through Hole φd (mm)
A: The number of Through Holes
T: Layer thickness of the board (mm)y: Net increase of surface area of the copper plating (mm2)When the diameter of a Through Hole is equal to the layer thickness, x=t, the net increaseof surface area of the copper plating, y, becomes the largest.

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Application Note
4.2 Setting of the Number of Through Holes
We evaluated the heat dissipation by changing the number of Through Holes. (We usedthe same conditions in the diameter of a Through Hole and the diameter of a set ofThrough Holes.)Table 3. Verification Item: Number of Through Holes
Reference Board

Number ofThrough Holes

Please refer to Figure 3 for the evaluation results of the heat dissipation depending on thenumber of Through Holes.
Reference Board
1

Evaluation of Heat Dissipation
TJ vs. The Number of a Through Holes

0.6W

0.9
Relative TJ (a.u.)Light Emitting Diode

Number of hole [N]

0.8
0.7
0.6
0.5
05
101520
The Number of Through-holes : N (piece)

25

Figure 3. Evaluation Result of Heat Dissipation Depending on the NumberThe measurement results in Figure 3 show that the higher the number of Through Holes,
the better the heat dissipation. We confirmed, however, that too many holes don’tnecessarily contribute to good heat dissipation. Moreover, too many holes could affect theperformance of the board and increase the processing cost.

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Jun. 9, 2021

Application Note
4.3 Setting of the Diameter of a Set of Through Holes
We evaluated the heat dissipation by changing the diameter of a set of Through Holes.
(We used the same conditions in the diameter of a Through Hole and the number ofThrough Holes.)Table 4 Verification Item: Diameter of a Set of Through Holes

Reference Board

Diameter of a set ofThrough Holes

Please refer to Figure 4 for the evaluation results of the heat dissipation depending on thediameter of a set of Through Holes.
Evaluation of Heat Dissipation
TJ vs. Diameter of a Set of a Through Holes

10.6W

0.9

Relative TJ (a.u.)Light Emitting Diode[φD]

0.8
0.7
0.6
0.5
051015
The Diameter of a set of Through-holes : φD (mm)20Figure 4. Evaluation Result of Heat Dissipation Depending on the Diameterof a Set of Through HolesThe measurement results in Figure 4 show that the heat can be dissipated more effectively
as the Through Holes are located nearer to the LEDs, as heat sources. It is necessary to takethe performance of the board and working efficiency into account to determine theoptimal location of the Through Holes.

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Application Note
5. Conclusion
Nichia recommends that the board be designed by taking the above evaluation results
into consideration.
The optimal thermal design can enable LED products to be used efficiently, leading to thequality improvement of the finished products.

Light Emitting Diode

Please note that the reverse side of the board can short circuit, when Through Holes arelocated on the copper plating with the potential energy.

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Application Note

Light Emitting DiodeDisclaimer
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This document contains tentative information, Nichia may change the contents without notice.
SP-QR-C2-210611-1
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