基于有限元仿真的环氧塑封可靠性研究

  随着电子信息技术的发展和社会的需要，电子产品逐渐向小型化、轻量化、高性能、低成本的方向发展，除了需要集成度更高的IC设计，电子封装技术在其中也发挥着重要的作用。根据所用材料的不同，封装可以分为：金属封装、陶瓷封装和塑料封装。近年来，塑料封装以其成本低、工艺简单、适合批量生产等优点，具有很强的生命力和市场适应能力。它自诞生以来发展迅速，现已逐步成为民用电子器件封装的主流。目前，环氧塑封料是最常用的塑封材料，在器件中的主要作用是为芯片及焊球提供保护，需要承受苛刻的多物理场环境应力。因此，塑封过程前后的可靠性考核至关重要，但可靠性试验往往是破坏性试验，成本较高。所以将可靠性试验与有限元仿真结合，可以说是一种探索封装结构失效内部机理的有效方案。

  本文将主要基于限元仿真的方法对环氧塑封封装结构的可靠性进行研究，主要包含两个方面：热应力的结构优化仿真以及工艺仿真过程中的模流仿真。对于热应力的结构优化仿真，解析了基于统计学理论的试验设计方法，同时采用该方法对选定的4因子2水平方案进行了分析，发现按照选定的高低水平，四个因子对于环氧塑封封装结构热应力均具有显著影响，分别是：环氧塑封料杨氏模量、基板杨氏模量、环氧塑封料CTE以及基板CTE。对于模流仿真，主要研究了转注成型工艺和底部填充工艺：在转注成型工艺中，选用液态下动态黏度较低、固化速率较慢的环氧塑封料，并根据其固化曲线设置合适的模具温度和充填时间将有利于模流充填；在底部填充工艺中，根据塑封结构的尺寸选择合适的点胶形式，并对底部焊球进行合理布局，适当留出芯片边缘尺寸，同时选用液态下动态黏度较低、润湿性较好的环氧塑封料，能够获得较好的充填效果。此外，设置合适的点胶路径长度能有效改善边缘效应。以上研究结果均可以作为实际塑封结构的材料选型及生产设计参考。

    With the development of electronic information technology and social needs, miniaturization, lightweight, high-performance, low-cost are the mainstream of electronic products, in addition to the need for a higher integration of IC design, electronic packaging technology is also playing an important role. According to the different materials used, electronic packaging can be divided into metal packaging, ceramic packaging and plastic packaging. In recent years, plastic packaging with its lower cost, simple process, suitable for mass production and other advantages, has a strong vitality and market adaptability. It has developed rapidly since its birth and has gradually become the mainstream of civil electronic device packaging. At present, epoxy molding compound is the most commonly used plastic packaging material, in order to provide protection for chip and solder ball, and it needs to withstand multi-physical stress. Therefore, the reliability assessment during the encapsulation process is very important, but most reliability tests are destructive, which result in high cost. Therefore, the combination of reliability test and finite element simulation can be said to be an effective method to explore the internal mechanism of the failure of packaging structure.

    This paper will mainly study the reliability of epoxy plastic packaging structure based on finite element simulation, which mainly includes two aspects: structural optimization of thermal stress simulation and mold flow simulation. For the structural optimization of thermal stress simulation, the design of experiment based on statistical theory was analyzed, and the selected 4-factor 2-level scheme was analyzed by using this method. It was found that according to the selected levels, the four factors all had significant influence on the thermal stress of epoxy plastic packaging structure, they were Young's modulus of epoxy molding compound, Young's modulus of the substrate, CTE of epoxy molding compound and CTE of the substrate. For the mold flow simulation, the transfer molding process and the bottom filling process were mainly studied. In the transfer molding process, the selection of epoxy molding compound with lower dynamic viscosity and slower curing rate under liquid state, and the appropriate mold temperature and filling time setting will benefit the mold flow filling. In the bottom filling process, appropriate dotting form, reasonable solder ball layout and chip edge size, and epoxy molding compound with lower dynamic viscosity and better wettability under liquid state can achieve better filling results. In addition, adjusting the dotting path length can effectively improve the racing effect. The research results can be used as references for material selection and production design of plastic packaging.

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