LED 封装可靠性研究

芯片产生的热量主要有三个导热路径，一是通过封装胶向环境散热，二是通过引线和引脚传热，三是通过固晶层和封装基板向下传热。其中绝大部分热量通过固晶层和封装基板传导。封装基板的主要性能围绕可靠性和散热性，就需要考虑热力学性能，因此要有高热导率和适当的热膨胀系数。

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The reliability of LED packaging is a typical light, heat, and force coupling problem, so its evaluation criteria revolve around optical properties, thermal properties, and mechanical properties. Various packaging materials and processes are involved in the performance and reliability of LEDs, including: light conversion materials, packaging adhesives, die-bonding materials, and packaging substrates.

01 light conversion material

In high-power LED packaging, phosphor materials are the most commonly used light conversion materials. According to the material, it can be divided into four series: rare earth garnet series, silicate series, nitrogen-containing compound series and sulfide series. Among them, the lead-acid yttrium aluminum garnet (Y3Al5O12) is currently the most widely used phosphor, commonly known as YAG phosphor. The particle diameter of the phosphor is usually 5-35 μm, and has the advantages of high brightness, wide emission peak, and low cost, but the excitation band is narrow, the spectrum lacks red light components, and the color rendering index is not high.

02 encapsulant

In the LED encapsulation process, epoxy resin or silicone is usually used as the encapsulant. However, because epoxy resin is prone to aging and yellowing, which seriously affects the light extraction efficiency, and silica gel has better light-thermal stability and high light transmittance, silica gel is often used as the encapsulating colloid in the packaging process. The light transmittance of silica gel to light of different colors can usually reach more than 97%. Increasing the refractive index of the LED encapsulant can effectively reduce the total reflection at the interface between the chip and the packaging material. Therefore, the packaging material with high refractive index and high light transmittance is also very important to improve the light extraction efficiency. At present, the main product comes from Dow Corning Corporation of the United States. The high-refractive-index silica gel launched by it has better photo-thermal stability and high light transmittance than traditional epoxy resin materials, so it can significantly improve the optical performance of LEDs. Scholars at home and abroad have also achieved the improvement of the quality of silica gel by developing or improving the process. Doping the high-refractive-index scattering particles in the encapsulant can increase the refraction index of the encapsulant and reduce the total reflection at the interface, thereby improving the light extraction efficiency.

03 Solid crystal material

Good heat dissipation is very important for high-power LED packaging. It is a prerequisite to ensure that LEDs can maintain high optical performance, and is an important part of achieving high reliability of LED packaging. The most important way to solve the heat dissipation of LED packaging is to use high thermal conductivity interface materials and substrate materials to reduce the thermal resistance of the chip to the external environment, and quickly conduct the heat of the chip to the external heat sink. Most of the heat generated by the LED chip is conducted downward through the die-bonding layer, so the die-bonding layer not only plays the role of fixing the LED chip, but also is the only path for heat transfer and is the key to heat dissipation. Therefore, it is necessary to reduce the heat of the die-bonding layer. resistance. The current mainstream method is to use high thermal conductivity solid crystal materials.

04 Package substrate

The heat generated by the chip mainly has three heat conduction paths, one is to dissipate heat to the environment through the packaging glue, the other is to transfer heat through the leads and pins, and the third is to transfer heat downward through the die-bonding layer and the packaging substrate. Most of the heat is conducted through the die-bonding layer and the package substrate. The main performance of the package substrate revolves around reliability and heat dissipation, and thermodynamic performance needs to be considered, so high thermal conductivity and appropriate thermal expansion coefficient are required.