LED封装荧光粉中光致发热的理论分析与实验研究

Since white LEDs are energy-saving and environment-friendly, and promising in market application, their inventors were awarded the Nobel physics prize in 2014. Nevertheless, thermal reliability is still a great challenge in both LED industry and academic, which is due to the fact that ~70% input electric power is converted into heat in LED products. So far, most studies on LED thermal management concentrate on chip heat generation and system-level cooling, and neglect the light-induced heat generation during phosphor conversion and the package-inside thermal management aimed at cooling phosphors. The recent investigation by the applicant shows that the light-induced heat generation in phosphors will result in the hotspot shift from chip to phosphors and even cause LED products’ failure. According to this discovery, we will establish the bridge between the energy loss in the microscale non-radiative electron transition and macroscale heat generation in phosphors, and build a light-heat coupled model based on micro/macro-scale methods in this proposal. Experimental study will be conducted to verify and improve this model, and to predict the heat generation. Based on the improved model, the effect of the phosphor concentration, shape and coating processes will be presented and used to verify new coating processes and control heat generation. This proposal covers a typical fundamental cross-disciplinary problem abstracted from the processes and applications. The research achievements are expected to instruct and realize some low thermal resistance and high-reliability packaging processes and promote the LEDs indeed penetrating the general lighting market.

由于白光发光二极管（LED）节能环保，市场应用前景巨大，发明人在短期内获得了2014年诺贝尔物理学奖。不过由于仍有大约70%的电功率转化为热量，热可靠性依然是LED挑战性问题之一。现有LED热管理研究大多关注的是芯片产热和系统级散热，忽视了荧光粉光致发光过程中的二次产热以及针对荧光粉的封装内热管理。申请人近期的研究表明：二次产热将会导致最高温度从芯片转移到荧光粉层，从而导致LED失效。目前对这一现象缺乏有效计算和分析手段，实验又无法准确测量。基于这一想法，本项目期望通过宏观和微观研究手段，搭建微观电子非辐射跃迁能量损失与宏观光致发热之间的桥梁，建立荧光粉光热耦合模型，结合实验来实现荧光粉发热的定量预测；研究不同荧光粉浓度、形貌和涂覆工艺等参数对荧光粉发热的影响规律，并进行工艺验证和热量控制。本课题是一个典型的从工艺和应用中提出的基础的交叉问题，期望研究成果能指导实现低热阻LED封装工艺。

发光二极管（LED）被誉为新一代绿色照明光源，在日常生活中已得到广泛应用。但仍有大约70%的电功率转化为热量，热可靠性依然是LED挑战性问题之一。现有LED热管理研究大多关注芯片产热和系统级散热，忽视了荧光粉光致发热及针对荧光粉的封装内热管理。目前对光致发热仍缺乏有效计算和分析手段，实验又无法准确测量。基于上述背景，本项目开展了以下四部分研究内容：（1）建立了荧光粉光热耦合模型。通过定量描述荧光粉层中的光输运过程建立了荧光辐射传递方程；通过Mie散射理论计算了荧光粉层的光学常数；通过谱元法求解了荧光辐射传递方程和通用边界，同时基于能量守恒定律和光致发光机理，计算了光致发热量，最后实验验证了模型的准确性。（2）探究了新型荧光粉温度预测模型和测试方法。提出了一种双向热阻网络模型，通过考虑芯片、荧光粉发热及所有的传热路径，实现了芯片和荧光粉温度的同时预测；开发了一种基于磁纳米颗粒的新型荧光粉温度测试方法，将磁纳米颗粒混合到荧光粉中进行涂覆，基于郎之万方程测试荧光粉温度。（3）分析了影响荧光粉发热的典型封装参数。基于光热模型研究了荧光粉浓度、厚度、量子效率、颗粒尺寸和封装形式对荧光粉光致发热量和荧光粉温度的影响规律，结果表明远离涂覆具有更低的荧光粉温度，增大热导率能大幅减低荧光粉温度。（4）开发了低荧光粉温度的封装工艺。基于模型预测结果，提出了掺杂透明高导热六方氮化硼颗粒工艺和荧光粉双面冷却工艺分别提高荧光粉层的热导率和散热能力，结果表明这两种工艺都能有效降低荧光粉工作温度。本项目对荧光粉光致发热和封装内热管理的基础问题进行了系统的研究，有助于指导实现低热阻高可靠性LED封装。