How to solve the LED heat dissipation problem

With a series of outstanding advantages such as energy saving, environmental protection, small size, long life, impact resistance, high reliability, and fast response speed, LED is considered to be a new type of green lighting source that replaces traditional lighting. With the development of LED applications in the lighting field, high-power, high-brightness, and high-quality LEDs have become an important development trend.

However, the current electro-optical conversion efficiency of LEDs is low. With the increase of power, the heat dissipation problem has become the biggest technical bottleneck for the popularization and development of LED lighting. How to improve the heat dissipation performance of high-power LED devices is one of the key technologies to be solved urgently on the road of its development.

A reasonable choice of packaging materials and packaging structures with efficient heat dissipation is an important part of effectively improving the reliability of high-power LED packaging. At the same time, designing and using more suitable heat dissipation structures and heat dissipation methods can also improve the reliability of high-power LEDs.

For the current high-power LED chips, the electro-optical conversion efficiency is only 20% ~ 30%, and the remaining 70% ~ 80% of the electric energy is converted into heat energy. The size of the LED chip is small, resulting in a high density of thermal power applied to the chip. If the generated heat accumulates in the chip and cannot be effectively dissipated, it will cause an increase in the junction temperature of the LED chip, leading to a series of problems such as increased light decay and shortened life. Generally speaking, in order to ensure the service life of high-power LED devices, the junction temperature of the chip is required to be controlled below 110 ℃. If the junction temperature is too high, the output luminous flux will be reduced, thereby affecting the light efficiency; the phosphor efficiency will be reduced, thereby affecting the color temperature. Generally speaking, in order to ensure the service life of high-power LED devices, the junction temperature of the chip is required to be controlled below 110 ℃. If the junction temperature is too high, the output luminous flux will be reduced, thereby affecting the light efficiency; the phosphor efficiency will be reduced, thereby affecting the color temperature. The junction temperature is a direct factor that affects the light efficiency of the LED. At room temperature, when the junction temperature increases by 1 ℃, the luminous efficiency decreases by 1%, and the luminous efficiency decreases almost linearly with the increase of the junction temperature. The relationship is shown in the figure below. When the junction temperature of the chip is increased from 25°C to 120°C, the luminous decay of the blue LED is about 10%, and the luminous decay of the white LED is about 30%. The light decay of amber LED even reached 80%. The lifetime of the LED decreases exponentially as the junction temperature increases. The operating life of LED chips at 30°C is about 20 times longer than that at 70°C. The increase in junction temperature from 40°C to 50°C will shorten the lifespan from 42,000 hours to 18,000 hours. Therefore, heat dissipation is a key issue affecting the development of LED.

The relationship between the luminescent efficiency and the junction temperature

How to solve the heat dissipation problem of high-power LED

1. Improve chip luminous efficiency

Improving the luminous efficiency of the chip can fundamentally reduce the heat generation in the LED chip. Improving the internal quantum efficiency and external quantum efficiency of the chip are the two basic starting points for improving the luminous efficiency of LEDs.

When a forward voltage is applied to the PN junction of the LED chip, a current will flow through the PN junction, and electrons and holes will recombine in the PN junction transition layer, thereby generating photons. However, not every electron-hole pair will recombine to produce a photon. This is because LED chips are impurity semiconductors, and there may be material quality, dislocations, and various defects in the process, leading to a series of problems such as impurity ionization, lattice scattering, and excitation scattering. It makes the electrons undergo a non-radiative transition when they transition from the excited state to the ground state, so no photons are produced. This part of the energy is not converted into light energy, but converted into heat loss in the chip PN junction.

To improve the internal quantum efficiency of the LED chip, it is necessary to study the manufacturing materials, the PN junction epitaxial growth process, and the light-emitting method of the chip's light-emitting layer. Through the unremitting efforts of researchers, the internal quantum efficiency has been significantly improved, from a few percent in the early days to several tens of percent. Nevertheless, there is still room for improvement in internal quantum efficiency in the development of LEDs in the future. 

In addition to the heat generated by non-radiative recombination, inside the chip, since the refractive index of the chip is much greater than that of silica gel, when photons emerge from the surface of the chip, the light path will be totally reflected, causing the photons to be reflected back into the chip, and finally absorbed and converted into heat. Outside the chip, high-power LED white light devices have a certain amount of energy loss during wavelength conversion when the phosphor is excited, and this part of the energy is absorbed and converted into heat. At the

same time, due to the unreasonable optical design in the packaging process, the light extraction efficiency of the LED will be reduced, and the un-emitted photons will also be converted into heat.

In order to improve the external quantum efficiency of LEDs, it is mainly considered from the perspective of the chip. The main technologies include: distribution Bragg reflector (DBR) structure, flip chip technology, substrate stripping technology, special-shaped chip technology, surface roughening technology, photonic crystal structure and transparent substrate technology. With the continuous improvement and maturity of these technologies, and the introduction of new processes and technologies, the external quantum efficiency of LEDs will be continuously improved.

2. Improve high-power LED packaging

Improving the packaging of high-power LEDs mainly considers two aspects, one is the packaging structure, and the other is the packaging material. Although the selection of packaging materials with good thermal conductivity is very important, the packaging structure has a considerable impact on the luminous efficiency and reliability of high-power LEDs.

In terms of packaging materials, materials with good thermal performance and excellent matching are mainly selected to make the thermal resistance on the heat flow channel smaller. For example: by selecting ceramic materials and alloy materials such as Cu/Mo, Cu/W as the substrate material to solve the problem of chip peeling and electrode lead breakage due to thermal expansion mismatch between the chip and the heat dissipation substrate; By selecting a suitable bonding material and determining the best thickness of the bonding material at the same time, the internal thermal stress of the chip can be reduced; By selecting sealing materials with better performance in all aspects, and at the same time, applying certain protection to the phosphors to improve the overall performance and reliability of high-power LED devices. By optimizing the packaging materials, the package thermal resistance of the device can be reduced to 4 ~ 10K/W, which can provide effective heat dissipation protection for LEDs with power up to 5W.

In terms of packaging structure, large-area chip flip-chip structure, metal circuit board structure, on-board packaging structure, heat conduction groove structure, microfluidic array structure, etc. can be adopted. Improving the package structure can not only improve the heat dissipation performance of the device from the perspective of reducing thermal resistance, but also increase the light extraction efficiency and reduce the number of photons converted into heat.

It can be seen that the reasonable combination of packaging materials and the optimized design of the packaging structure are conducive to the improvement of high-power LED packaging, which is extremely important for improving the heat dissipation performance of high-power LEDs, and provides a new way to solve the heat dissipation problem of high-power LEDs.