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晶鑫光电/JINGXING Opto-electronic

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晶鑫案例

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LED 封装可靠性研究

LED 封装可靠性是典型的光、热、力耦合问题,因此其评价标准围绕光学性能、热学性能和力学性能等方向。在 LED 性能及可靠性中涉及各种封装材料和工艺主要包括:光转换材料、封装胶、固晶材料、封装基板。

01光转换材料

在大功率 LED 封装中,荧光粉材料是最常用的光转换材料。其按材料分可分为稀土石榴石系、硅酸盐系、含氮化合物系和硫化物系四大系列。其中铅酸盐的钇铝石榴石(Y3Al5O12)是目前使用最广泛的荧光粉,俗称 YAG 荧光粉。该荧光粉的颗粒直径通常在5~35μm,具有亮度高、发射峰宽、成本低的优点,但激发波段窄,光谱中缺乏红光的成分,显色指数不高。

02封装胶

在 LED 封装过程中,通常采用环氧树脂或硅胶作为封装胶。但由于环氧树脂容易出现老化变黄,严重影响出光效率,而硅胶具有更好的光-热稳定性,透光率也很高,所以在封装过程中常用硅胶作为封装胶体。硅胶对不同颜色的光透光率通常能达到97%以上。提高 LED 封装胶折射率可有效减少芯片和封装材料界面上的全反射,因此高折射率、高透光率的封装材料对于提高取光效率也很重要。目前主要产品来自美国道康宁公司,其推出的高折射率硅胶比传统的环氧树脂材料具有更好的光-热稳定性,透光率也很高,因而能显著改善 LED 的光学性能。国内外学者通过开发或者改良工艺,也实现了硅胶质量的提升。在封装胶中掺杂高折射率的散射粒子,可以提升封装胶的折射率,减少界面全反射,从而提升取光效率。

03 固晶材料

良好的散热对于大功率 LED 封装至关重要,是保障 LED 能够维持高光学性能的前提条件,是实现 LED 封装高可靠性的重要内容。解决 LED 封装散热的最主要的方法是利用高导热的界面材料和基板材料降低芯片到外部环境的热阻,将芯片的热量快速传导到外部的散热器中。LED 芯片产生的绝大部分热量是通过固晶层向下传导,因此固晶层不仅起到固定 LED 芯片的作用,而且是热量传递的必经路径,是散热关键,因而需要降低固晶层热阻。目前采用的主流方法为采用高导热率的固晶材料。

04 封装基板

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.