LED Strip Heat Dissipation Performance based of LED pitch and base material

LED strips and modules used for lighting fixture use, in general, multiple LEDs.

As operating a single LED generates heat, more is generated when multiple LEDs are mounted on a PCB, due to the mutual effect. As such, in the case of LED strips or modules, the junction temperature (TJ) of each LED gets higher, compared to a single light source. This leads to the decrease in the LEDs lifetime and luminous flux.

For LED strips and modules, a better thermal management is required to minimize TJ and allow a longer lifetime of the installed products. For this purpose, the LED pitch, the PCB base material and the use of aluminum profile must be taken in consideration.

LED pitch, input power and PCB base material

To demonstrate the effects on thermal management of LED pitch, input power and PCB base material, several LED modules with 9 x Nichia 757 LEDs where used for testing.  

Nichia 757 LED specs	NS2W757A-V1	NF2W757AR-V1
Dimensions [mm]	3.0×3.0×0.52
Forward Current [mA]	65	150
Maximum Forward Current [mA]	180	200
Forward Voltage [V]	2.85	6.3
Maximum Junction Temperature [°C]	120

 

 

The square modules have 9 LEDs with 5 mm, 6 mm, 7 mm and 10 mm LED pitch, respectively. For a LED strip this relates to 200 to 100 LEDs per meter:

LED pitch	Strip LEDs/m
5mm	200
6mm	166
7mm	140
10mm	100

 

One batch have a FR-4 PCB while the other Aluminium PCB (AL). For all modules the TJ was measured at various power levels values: 1.7 W, 4.1 W, 5.7 W and 8.5 W.

Low power test (1.7 W & 4.1 W)

The first test was made at power levels typical for LED strips and modules for ambient lighting (max 1000 lm/m).

Figure below shows the TJ measurement results at 4.1W (IF=150 mA) and 1.7W (IF=65 mA) for each pitch value. On the FR-4 LED module batch, the LED pitches did not affect the TJ much, when the input powers were low. However, when the input power was increased, the TJ jumped significantly, with some difference depending on the LED pitches. The smaller the LED pitch was, the higher the TJ became. On the other hand, there was no difference in the TJ values on the AL PCB batch, regardless of the LED pitches.

High power test (5.7 W & 8.5 W)

The second test was made at power levels typical for LED strips and modules for general lighting (max 2000 lm/m).

Figure below shows the TJ measurement results at 8.5W (IF=150 mA) and 5.7W (IF=100 mA) based on the test pitches and base material. The measured TJ values of the FR-4 batch show the limitation of this material for high brightness LED strips with small LED pitch. Even at 10 mm LED pitch, the TJ of the LEDs is at the maximum allowed value.

 

At 5mm pitch, it is so far above, that rapid thermal damage to the LEDs will occur:

This test explains why the LED strips we manufacture have a maximum pitch of 7mm and use high efficacy LEDs to maximize light output while keeping the power low.  This is radically different from the approach of many others manufacturers, especially those from China. They choose to use many low cost, low efficacy LEDs for the high power LED strips with 1500 lm/m or more. Strips with 240 LEDs/m (4.1mm LED pitch) or more are very common.

Our LED strips have very long lifetime, 50.000h or more at L70 for exactly this reason, while very high density low cost LED strips barely last a few thousand hours.

The test also shows the difference an AL PCB makes, regardless of the LED pitches the TJ was well below the maximum. 

Aluminum case and impact on LED strip performance

Most white LEDs emit white light from the combination of blue LED die and yellow phosphor. As the LED die generates higher temperature, the light output from the die is reduced, decreasing the output excited by phosphor. Therefore, the higher the junction temperature is, the lower the luminous flux becomes.

Customers may not be able to realize the designed luminous flux, if the junction temperature is high. Moreover, when there is variation in the junction temperatures among the LEDs, the emission light from the LED strip will not be even.

Customers are required to optimize the thermal design for the LED strips by making it possible to dissipate heat via an aluminum profile.

To showcase this case, the experiment measured the heat distribution over the 140 LED Strip and the LEDs’ junction temperature with/without heat dissipation system through an aluminum profile. A thermograph was used to evaluate the heat distribution.

The LED strip luminous flux was measured just after the LEDs were operated and while the temperature was saturated, and then, those values were compared to obtain the depreciation rate.

Table below shows the measurement results. The luminous flux depreciation rate was calculated by the luminous flux measured just after the LEDs were operated and while the temperature was saturated.

For each LED strip, the heat was uniformly distributed all over the board, regardless of the existence of the heat sink. Therefore, there was no problem in the heat dissipation in either cases; however, the LEDs’ junction temperature without the heat sink was at least 10°C higher than the LED strip in the aluminum profile. In addition to the impact on LED lifetime, the LED strip without the heat sink had a luminous flux depreciation rate of at least 50% higher.

The findings show that decreasing the junction temperature contributes to minimizing the decrease of the lamp’s luminous flux.

We usually recommend that flexible LED strips with 100 LEDs or more per meter are installed inside an aluminium profile.