Because of its high brightness, low heat, long life, non-toxic, recyclable and other advantages, LED is known as the most promising green lighting source in the 21st century. Statistics show that if 50% of China's light sources are replaced with LEDs, it will achieve an annual power saving of 210 billion kWh, which is equivalent to building 2.5 more Three Gorges projects. At the same time, it also has unparalleled advantages in terms of life span, application flexibility, etc., and will become the inevitable development direction of future lighting.
The principle of LED luminescence is to directly convert electrical energy into light energy. Its electro-optical conversion efficiency is about 20%-30%, and its light-to-heat conversion efficiency is about 70%-80%. With the reduction in chip size and the substantial increase in power, the LED junction temperature remains high, causing a series of problems such as reduced light intensity, spectral shift, increased color temperature, increased thermal stress, accelerated aging of components, etc. Greatly reduces the service life of LED. Junction temperature is also an important technical indicator to measure the heat dissipation performance of LED packages. When the junction temperature rises above the maximum allowable temperature (150℃), high-power LEDs will be damaged due to overheating. Therefore, in high-power LED packaging equipment, heat dissipation is the bottleneck restricting its development, and it is also a key problem that must be solved.
At present, the global LED industry solves the heat dissipation problem from three aspects, one is the packaging structure, the other is the packaging material, and the heat dissipation substrate. The fundamental problem can be solved only by dissipating heat.
At present, there are three main packaging structures used in heat dissipation:
1. Flip chip structure
In the traditional formal chip, the electrode is located on the light-emitting surface of the chip, so it will block part of the light and reduce the light-emitting efficiency of the chip. At the same time, the heat generated by the PN junction of this structure is exported through the sapphire substrate. The thermal conductivity of sapphire is low and the heat transfer path is long. Therefore, the chip of this structure has a large thermal resistance and the heat is not easily dissipated. From an optical and thermal perspective, this structure has some deficiencies. In order to overcome the shortcomings of front mounted chips, Lumileds Lighting developed a flip chip in 2001. In the chip of this structure, the light is taken out from the sapphire on the top, which eliminates the shading of the electrode and the lead, and improves the light extraction efficiency. At the same time, the substrate uses a high thermal conductivity ceramic circuit board, which greatly improves the heat dissipation effect of the chip.
2. Micro-spray structure
In this packaging system, the fluid in the fluid cavity forms a strong jet at the series of micro-jets under a certain pressure. The jet directly hits the lower surface of the LED chip substrate and takes away the heat generated by the LED chip. Under the action, the heated fluid enters the small fluid cavity to release heat to the external environment, so that its temperature drops, and flows into the micropump again to start a new cycle. Similar to the water-cooled heat dissipation of the computer motherboard, this micro-jet structure has the advantages of high heat dissipation and uniform temperature distribution of the LED chip substrate, but the reliability and stability of the micro-pump have a great impact on the system, and the system structure is more complicated Higher operating costs.
3. Thermoelectric cooling structure
A thermoelectric cooler is a semiconductor device whose PN junction is composed of two different conductive materials, one carries a positive charge and the other carries a negative charge. When current passes through the junction, the two charges leave the junction area and carry There is heat to achieve the purpose of cooling. Similar to the principle of air conditioning and refrigeration. However, the technology is not yet mature and cannot be applied in batches.
It can be seen from the above that the overall heat dissipation effect of the LED has a great relationship with the heat dissipation between the interfaces, so the thermal resistance of a certain interface can be reduced to improve the heat dissipation effect. Encapsulation material is one of them. The packaging material is mainly the substrate and glue. Currently, LED packaging glue commonly used thermal conductive adhesive, conductive silver adhesive and the latest fluorescent ceramic adhesive.
1. Thermally conductive adhesive
The main component of the commonly used thermal conductive adhesive is epoxy resin, so its thermal conductivity is small, thermal conductivity is poor, and thermal resistance is large. In order to improve its thermal conductivity, the substrate is usually filled with high thermal conductivity materials such as aluminum oxide, boron nitride, silicon carbide, etc. Thermal conductive adhesive has the advantages of insulation, thermal conductivity, shockproof, easy installation, simple process, etc., but its thermal conductivity is very low (generally less than 1w/mk), so it can only be applied to LED packaging devices with low heat dissipation requirements. If the LED power is too large, the thermal conductivity cannot keep up with the demand, and a large amount of junction temperature will also be generated.
Second, conductive silver glue
Conductive silver paste is a GeAs, SiC conductive substrate LED, with red, yellow, yellow-green chip LED package dispensing or key packaging materials in the back electrode process, with fixed bonding chip, conductive and thermal conductivity, transmission The effect of heat has an important influence on the heat dissipation, light reflectivity and VF characteristics of LED devices. But the technology is not very mature, the cost is high, and it is not popular.
Fourth, fluorescent ceramic glue
With the best optical quality, highest performance, high hardness, corrosion resistance, high temperature resistance and other characteristics. It is a new technology, much better than traditional conductive adhesive, but it is still in the experimental stage and has not yet been formed.
Through the above analysis, it can be seen that the real heat dissipation interface materials are basically still in development, and the application will take some time. A certain heat dissipation route of the LED package device is from the LED chip to the bonding layer to the internal heat sink to the heat dissipation substrate and finally to the external environment. It can be seen that the heat dissipation substrate is important to the heat dissipation of the LED package, so the heat dissipation substrate must have the following characteristics: high Thermal conductivity, insulation, stability, flatness and high strength.
1. Metal substrate
The metal substrate is bonded to the metal with a high thermal conductivity (copper, aluminum, etc.) on the original printed circuit board, so as to improve the heat dissipation effect of the electronic device. It is a key link to connect the internal and external heat dissipation channels. The advantage of the metal substrate is that the cost is relatively low and it can be mass-produced, but it also has certain disadvantages: ① low thermal conductivity, and the thermal conductivity of MCPCB can reach 1-2.2W/(m K). ② The thickness of the insulating layer in the metal substrate structure should be moderate, neither too thick nor too thin. Too thick an insulating layer increases the thermal resistance of the entire metal substrate and affects the heat dissipation effect; if the insulating layer is too thin, if the voltage applied to the metal substrate is too high, it will break down the insulating layer and cause a short circuit.
Second, the ceramic substrate
The sintered ceramic substrate has good heat dissipation, high temperature resistance, humidity resistance, breakdown voltage and breakdown voltage, and its thermal expansion coefficient is well matched, which has the characteristics of reducing thermal stress and thermal deformation. Therefore, ceramic substrates have become an important substrate material in high-power LED packages. At present, the most commonly used ceramic materials are alumina, aluminum nitride, beryllium oxide, silicon carbide, etc.
in conclusion:
1) In the high-power LED packaging devices, to achieve the purpose of low thermal resistance and fast heat dissipation, packaging materials have become the key technology. It is a hot topic in the future to strive to find better packaging materials to improve the heat dissipation of LED packaging devices.
2) To solve the heat dissipation problem of high-power LED packaging devices, it is necessary to select appropriate packaging materials (including thermal interface materials and substrate materials, etc.). In the process of selecting thermal interface materials and substrate materials, appropriate materials should be selected according to the appropriate occasions. The common thermal interface material used in high-power LED packages is conductive silver paste, and the commonly used substrate is ceramic circuit boards.