BGA结构无铅微焊点在电-热-力多场作用下的迁移和失效行为及其尺寸效应

As electronic packaging technologies continue to move towards high density, small-scale and the increased structural complexity, the dimension of lead-free solder interconnects has been scaled down gradually; and this has led to increasingly servere serive conditions of the interconnects under thermal, electric and mechanical loadings etc, and consequently brought about serious concerns about structural integrity and reliability of packaging systems due to the degradation of microstructure and properties of micro-scale lead-free solder interconnects. The present project aims to investigate systematically the migration behavior, evolution of interfacial microstructures and micro-defects, degradation of properties and failure mechanisms of ball grid array (BGA) structure micro-scale lead-free solder interconnects under electro-thermo-mechanical coupling fields, through experimental characterization, phase field simulation, numerical modeling and theoretical anaylysis; further, the size effect on the above microcharacteristics and properties as well as failure mechanism of the interconnects will be taken into account. The studies will focus on revealing the physical characteristics, solid metallurgical process and mechanics mechanism of migration and failure of micro-scale solder interconnects under multiple fields; understanding the fundamental trends of the evolution of compositions, interfacial microstructure and micro-defects, as well as their influence on properties and reliability of increasingly miniaturized solder interconnects; establishing a predictive model for evaluating the migration lifetime and critical conditions of failure of micro-scale solder interconnects under thermal, electric and mechanical loadings; developing the reliability assessment methodology for micro-scale lead-free solder interconnects under severe usage conditions. It is expected that the above studies will provide theoretical background for design of new lead-free solders and high-performance solder joints/bumps as well as for prediction of properties of solder interconnects, and offer a theoretical guide for reliability and durability assessment of lead-free electronic packaging structures and systems.

电子封装技术持续向高密度、微小化和结构复杂化发展导致新型无铅互连焊点尺寸日益减小及承受越来越复杂的电、热和力等载荷共同作用，由此引发的无铅微焊点组织异变和性能退化带来了严峻的互连结构完整性及系统可靠性问题。本项目拟以实验物理表征为主并结合相场法和数值模拟及理论分析系统研究BGA结构无铅微焊点在"电-热-力"场作用下的迁移行为、界面组织和缺陷演化、性能退化和失效规律及其尺寸效应，全面揭示微焊点在多负载场作用下迁移和失效的物理本质和固相冶金机理及力学机制；掌握无铅微焊点尺寸微小化过程中成分、界面组织和缺陷演变特征及其对性能和可靠性影响的基本规律, 建立微焊点在"电-热-力"场作用下迁移寿命及临界失效的预测模型，发展出无铅微焊点在复杂服役条件下的可靠性评价方法，为新型无铅钎料设计、高性能微互连焊点结构设计和性能预测提供技术基础理论支持，并为无铅电子封装结构和系统的可靠性与耐久性评价提供理论指导。

本项目采用实验、理论分析和数值模拟相结合的方法对无铅微焊点的形成过程和服役可靠性进行了系统研究。主要研究内容包括微焊点形成过程中界面冶金反应行为及界面金属间化合物（IMC）组织演化规律、微焊点服役时界面区域Kirkendall空洞形成和形貌演化及长大过程、焊点结构和显微组织不均匀性对电迁移行为的影响、典型载荷下微焊点力学行为及疲劳性能的尺寸效应及其本质、电-热-力耦合场下的力学行为。本项目取得主要研究结果和创新之处包括：首次采用DSC-回流控温时效处理工艺制备了局部熔化的单界面BGA结构微焊点，研究了焊点界面IMC净生长行为和界面Cu基底的溶蚀行为，研究指出界面Cu6Sn5层和Cu3Sn层生长分别主要受晶界扩散和体积扩散控制；采用相场法模拟研究指出界面反应初期IMC形貌特征的演化主要受控于晶界扩散系数和IMC相与液体钎料间的界面能，并全面认识和阐明了IMC层中Kirkendall空洞形成和形貌演化过程，剖析了影响Kirkendall空洞产生和长大的材料、工艺和服役条件等因素；对比研究了具有结构和显微组织不均匀性微焊点中的电迁移行为，揭示了电迁移与显微组织演化间的相互作用规律，指出了焊点结构和显微组织不均匀性对电流密度大小和分布有显著影响，且显微组织不均匀性对电流密度的影响程度甚至可以超过焊点结构不均匀性；全面而深入地从线弹性力学、粘塑性力学、断裂力学和损伤力学角度研究了微焊点在典型载荷下力学性能及疲劳行为尺寸效应的本质，改进和发展了基于损伤力学的微焊点低周疲劳寿命预测模型，并运用发展的模型对典型结构微焊点的疲劳寿命进行了准确的评估及预测；创新设计和搭建了目前可能属于国内外唯一的最接近工程应用实际情况的微焊点“电-热-力”耦合加载实验系统，并系统研究了微焊点在电-热-力耦合场下力学行为及断裂特征，引入了损伤因子和焦耳热成功解释了电-热-力耦合场促进微焊点变形的作用机制，阐明了电流密度、温度、应力及其作用时间以及焊点尺寸对焊点变形和断裂行为的影响规律。本项目所发展的研究方法论及获得的研究成果把无铅微焊点的制备可靠性和服役可靠性方面的基本认识向前推进了一大步，为无铅钎料选择和设计、微焊点设计和制备以及封装钎焊后微焊点的使役性能和可靠性评价等提供了重要理论指导，为今后尺度持续微小化的无铅微互连焊点及封装系统的性能预测和可靠性评价提供了具有重要参考价值的量化方法和理论依据。

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