多场耦合混装焊点的可靠性及其失效机制研究

With the transition from traditional Sn-37Pb solder to Pb-free solder, the mixed solder joint by soldering Pb-free components with Sn-37Pb is inevitably occurred in defense electronic industry. This project is intended to make a deep and systematical research on the major scientific issues on the reliability of micro-ininterconnected mixed solder joints under coupling effects from multi-fields applications. The behaviors on structural or compositional mixed solder joints under current stressing, thermcal cycling and random vibrationwill be studied under thermo-electric and mechanical-electric coupling effects. The reliability model, mechanical behaviors and failure mechanism on mixed solder joints under multi-fields will be illustrated. Firstly, the effect of reflowing conditions on microstructure, structure and constitutive equation of mixed solder joints is studied with consideration of size effect on solder. The controlled method on the optimization of reflowing processes for mixed solder joints is provided. Secondly, the reliability of mixed solder joints is studied under the effect of single field. The prediction on fatigue life, crack propagation and failure mode on mixed solder joints will be built for thermcal cycling, random vibration and electromigration, respectively. Finally, studies on the reliability and failure mechanism of mixed solder are finished under thermo-electric and mechanical-electric coupling effects. With the 3D computed tomography using serial-sectioning technique, relationship between the microstructural evolution and the micro damage in the solder joints is built during the initialization and development of fatigue crack under tests.The effct of microstructure of solder joints on the fatigue crack is researched. The failure mechanism on the solder joints will be discovered based on the relationship between macro crack and micro damage in the joints.

基于国防电子产品目前有铅-无铅微互连混装的实际应用需求，本项目针对微互连混装焊点在多环境耦合场作用下的可靠性这一科学问题，研究混装焊点在热电和力电耦合作用下的电应力行为、热循环可靠性、随机振动可靠性，建立微互连混装焊点在多场耦合下服役可靠性模型，并揭示微互连混装焊点的力学行为及失效机制。首先研究了回流焊工艺对混装焊点组织、结构、本构方程及其尺寸效应行为，并提出适合于有铅/无铅混装的回流焊工艺的控制方法。其次研究混装焊点在单场条件下的可靠性，建立热循环、随机振动、电迁移条件下焊点的疲劳寿命模型、裂纹扩展机制和焊点失效机理。最后过渡到混装焊点在多场耦合作用下的可靠性研究。基于3D断层成像技术分析微混装焊点热、力、电等多场耦合作用下焊点裂纹的扩展，探索焊点微结构组织演化和损伤过程的影响，研究焊点微观组织及微观损伤与疲劳裂纹之间的联系，建立基于宏观裂纹和微观损伤之间的焊点失效机制。

国防电子产品的长期可靠性要求以及无铅技术的逐渐推广使得国防电子产品在现阶段必须要面临的问题就是有铅无铅混装或者低温封装工艺，该混装工艺主要是指利用低熔点的Sn-Bi或Sn-Pb焊膏将Sn-Ag-Cu的Bump回流焊接在PCB板上。这一封装过程包含着组织、力学性能、可靠性的演变，介于此，混装焊点的可靠性问题日益成为研究焦点。本项目针对微互连混装焊点在多环境耦合场作用下的可靠性这一科学问题，研究混装焊点在热电和力电耦合作用下的电应力行为、热循环可靠性、随机振动可靠性，建立微互连混装焊点在多场耦合下服役可靠性模型，并揭示微互连混装焊点的力学行为及失效机制。.通过对不同焊接工艺形成的结构混装焊点和成分混装焊点基体组织进行了组织分析，研究了温度和时间对混装焊点组织形貌及焊点结构上的影响；利用结合强度测试仪对混装焊点进行了推剪试验，研究了回流温度和时间对混装焊点力学性能的影响。通过回流工艺控制混装结构、减少微混装焊点内部缺陷，提高其钎焊质量及其可靠性。.研究了混装焊点的可靠性。针对焊点振动可靠性建立了基于Miner累计损伤疲劳理论、Masson-Coffin方程以及三带技术的焊点可靠性量化评价指标，同时通过改变不同工艺参数对焊点的振动可靠性进行了评价与优化。针对焊点热循环可靠性建立了焊点的疲劳寿命模型，研究了焊点微观组织及微观损伤与疲劳裂纹之间的联系，建立基于宏观裂纹和微观损伤之间的焊点失效机制。建立了界面金属间化合物层的生长动力学模型，评估了热循环和等温时效对组织与焊点力学性能的影响，引入了界面IMC的生长加速因子。针对焊点电迁移可靠性，Sn-Bi焊点内Bi原子会朝向阳极迁移并形成明显的富Bi层，且焊点内的Bi相会发生明显的粗化现象。热电耦合会提高Bi原子的迁移速度和DZ*。Sn-Ag-Cu焊点将产生显著的极化、晶粒旋转引起的晶界的凸起和凹陷并降低焊点拉伸强度。混装可以显著提高焊点电迁移可靠性。