Optimizing Heat Dissipation on LED Substrates | Cu Area and Thermal Via Experiment Report | NEXTY Electronics, a trading company for electronic components and semiconductors

For all you engineers out there who are struggling with LED heat dissipation design, do you ever find yourself wondering, "I'm not sure how effective Cu areas (copper foil patterns) and thermal vias are?"

In this column, we will use the OSCONIQ P3030 LED from ams-O STATIC RAMS as an example to provide an easy-to-understand explanation of everything from the basics of LED heat dissipation design to the size of the Cu area and changes in junction temperature (Tj) due to thermal vias.

This column is recommended for the following people:

Those who want to learn the basics of LED heat management
Those who want to know the effects of Cu area and thermal via design
Those looking for design tips that can be used in practice

Please read to the end to learn how to maximize the performance of your LEDs and extend their lifespan.

Evaluation target and main characteristics

The LED used in this evaluation was the OSCONIQ P3030 (GW QSSPA1.EM) LED from ams-OSMRAM.
The OSCONIQ P3030 offers excellent cost performance and is used in applications such as indoor lighting and street lighting.
Table 1 summarizes the main characteristics.

Table 1. Characteristics of GW QSSPA1.EM

Maximum rating		symbol	min	typ	max	unit
	operating temperature	Top	-40	~	125	°C
	Junction Temperature	Tj		~	135	°C
	Maximum forward current @Tj=85°C	IF	100	~	1300	mA
Electrical and Optical characteristics		symbol	min	typ	max	unit
	Forward voltage	VF	2.7	2.85	3.2	V
	Color rendering index	CRI	80	83
	Thermal resistance @η=40%	RthJS elec.		5.4		K/W
	luminous flux	Φv	104		164	lm
	color temperature		2200		5000	K
	Sorting conditions: IF=350mA, Tj=85°C

OSCONIQ P3030 structure and heat dissipation design points

Figure 1 is the dimensional drawing listed on the data sheet.
The structural features are as follows:

Die 1: ESD protection DIODES
Die 2: LED chip
Back: 3-pad configuration (anode and cathode)

The LED chip is placed on the center pad, which is designed to be larger than the other two pads to dissipate heat efficiently.
Therefore, increasing the Cu area of the center pad is key to improving the heat dissipation of the LED.

Figure 1. Dimensional drawing of the GW QSSPA1.EM

Solder Pad Design Basics

The first thing to check when designing LED heat dissipation is which TERMINALS the LED chip is mounted on.
In this OSCONIQ P3030, the LED chip is located on the central pad.
Figure 2 shows the solder pads listed in the datasheet.

Solid line: Footprint (where the LED is mounted)
Dotted line: Cu area (copper foil area for improved heat dissipation)

The footprint has dimensions for the mounting position, but the dimensions of the Cu area are not listed.
This is because it must be adjusted arbitrarily according to the input power and thermal design conditions.

Key points:

Increasing the central Cu area improves the heat dissipation of the LED.
In particular, the central pad is located directly under the LED chip and is essential for heat dissipation, so it is essential to secure a Cu area when designing the board.

Tj temperature evaluation

When designing heat dissipation for an LED, a difficult question is how much Cu area should be secured in the center pad.
Therefore, this time we used the OSCONIQ P3030 to evaluate the effect of the Cu area size and the presence or absence of thermal vias on the junction temperature (Tj).

Evaluation board specifications

The specifications of the board used for evaluation are as follows (see Figure 3):

Size: 70 x 70mm x 1.6mm
Base material: FR-4
Copper foil thickness: 35μm
LED position: Center of the board
Single-sided board

There are four types of Cu areas available.

4 × 4 mm
8 × 8 mm
16 × 16 mm
32 × 32 mm

*Cu area is the area on one side.

Furthermore, three types were prepared to confirm the effect of thermal vias (see Figure 4).
The Cu area is 16 x 16 mm.

1 line
2 lines
3 lines

*Thermal via diameter: 0.35 mm
*When verifying thermal vias, the same Cu area as on the front side is spread on the back side.

Figure 3: Evaluation board specifications
Figure 4: Evaluation board specifications

Measurement conditions

The measurement conditions are as follows:

IF: 700mA (2W)
Ta: 23℃
Power-on time: 5 minutes

How to measure Tj

The measurement method is as follows:

1. Attach a thermocouple to the Ts point as shown in Figure 5.
2. Installing the LED substrate in the integrating sphere
3. Turn on the LED
4. Optical measurement/Ts temperature measurement after 5 minutes

After 5 minutes, measure the radiant flux and estimate Tj using the formula below.

Tj = Ts + RthJS_real × Pheat
= Ts + RthJS_real × (IF × VF − Φe) ・・・①

Ts: Solder point
RthJS_real : Thermal resistance
Pheat: Heat generation
IF: Forward current
VF: Forward voltage
Φe: radiant flux

The datasheet for GW QSSPA1.EM lists RthJS elec., but this time we will use RthJS_real.
For the differences between the two, please refer to our previous technical column.

LED glossary

RthJS_real for GW QSSPA1.EM can be calculated as follows:

RthJS_real =RthJS_elec. / (1−ηe)
=5.4/(1−0.4)
=9K/W・・・②

ηe: luminous efficacy

Measurement results

Figure 6 shows the results of the change in Tj depending on the size of the Cu area.

4 x 4mm: Tj exceeds 135℃, insufficient heat dissipation makes it unsuitable
8 x 8mm -> 16 x 16mm: Tj significantly reduced by expanding the Cu area
16 x 16mm -> 32 x 32mm: There is an effect, but the improvement is small

Conclusion:
The larger the Cu area, the greater the effect, but the effect saturates at 32 x 32 mm or more. Excessive expansion of the Cu area leads to increased costs, so the optimal size is thought to be around 16 to 32 mm.

Figure 6: Results of Tj transition depending on Cu area size

Figure 7 shows the results of the transition of Tj according to the number of thermal vias.

No thermal via: High Tj
1 line added: A clear improvement
Adding 2-3 lines: Minimal effect

Conclusion:
Inserting thermal vias is effective in lowering Tj, but increasing them too much only has limited effect.
Considering the balance between cost and effectiveness, we recommend placing at least one line.

Figure 7: Results of Tj transitions by number of thermal vias

The lifespan of an LED depends on temperature and current.
Higher temperatures and currents shorten the LED's lifespan.
For example, if there is an LED with a lifespan of 10,000 hours at IF=700 mA and Tj=85°C, it is essential to keep the Tj of the LED below 85°C in order to achieve 10,000 hours as a light source system.
The results of this evaluation show that a Cu area of approximately 32 x 32 mm is required.
Furthermore, in harsh environments where the ambient temperature is high, heat dissipation can be significantly improved by using a metal substrate such as aluminum or copper.
This ensures the lifespan of the LED and improves the reliability of the entire light source system.

*If you need the LED life curve, please contact us.

Summary

The following points are important for improving the heat dissipation of LEDs.

Expansion of Cu area
Increasing the Cu area of the central pad where the LED chip sits helps with heat dissipation.
However, if it is too large, the effect will saturate, so the optimal size is around 16 to 32 mm.
Adding thermal vias
By providing thermal vias, heat can be transferred to the back side of the board.
We recommend placing at least one line, but the effect will be limited if you increase it too much.

In this evaluation, we confirmed the change in Tj depending on the size of the Cu area and the presence or absence of thermal vias.
As a result, we found that expanding the Cu area provides a high heat dissipation effect, but the effect saturates, and that while adding thermal vias also improves heat dissipation, excessive placement is unnecessary.

We can handle optical measurements, Tj measurements, and thermal simulations.
If you have OTHERS problems with LEDs, please feel free to contact us.