结构和组织不均匀微凸点焊点在电-热-力耦合场下的蠕变行为研究

Micro-bump interconnect is one of the key techniques for 3D chip stacking integration, which can meet the requirement of the increasing high density of integrated circuits and electronic devices. However, the inhomogeneities of micro-bump solder joint configuration and microstructure cause loadings distributing inhomogeneously, and the features of loadings distribution vary with the microstructural evolution, which can result in serious deterioration in joint reliability. This project aims to systematically investigate the creep behavior of micro-bump solder joint with inhomogeneous configuration and microstructure under electro-thermo-mechanical coupled loads, through experimental characterization, including a unique and key technique on detecting electric current density distribution using MFM, assisted with cellular automata and finite element coupling simulation method. The focus will be placed on detecting the inhomogeneous distribution of electric current density induced by the inhomogeneities of joint configuration and microstructure, as well as getting a knowledge of the morphology and crystallographic characteristics of the major phases; then uncovering the features of inhomogeneous distribution of electro-thermo-mechanical coupled loads; understanding the fundamental trend of the interacting influence between microstructural evolution and variation of the distribution of coupled loads, constructing a quantitative relationship between characteristics of microstructural evolution and variation of the distribution of coupled loads; revealing the physical characteristics and micromechanism of microstructural evolution and anti-creep property degradation as well as fracture behavior of joints, establishing the creep constitutive model incorporating with the inhomogeneities of joint configuration and microstructure as well as the coupled loads. It is expected that the above studies will provide theoretical reference for manufacturing process optimization, reliability and durability assessment as well as failure analysis of micro-bump solder joints.

微凸点技术作为三维芯片集成封装的关键之一有助于解决集成电路和电子器件高密度化问题，但微凸点焊点的结构和组织不均匀性致使服役时焊点所受载荷呈非均匀分布，且载荷分布特征随组织演化而发生改变并严重影响焊点可靠性。本项目以磁力显微镜量化测算电流密度分布技术作重要依托的实验研究为主并辅以元胞自动机与有限元相结合的模拟，系统地研究结构和组织不均匀微凸点焊点在电-热-力耦合场下的蠕变行为，探明焊点的结构和组织不均匀性所致电流密度不均匀分布状况及各物相形貌和晶体学信息，揭示焊点中耦合场非均匀分布特征，掌握焊点微观组织演化与耦合场分布变化之间交互影响的规律，并建立这两者的特征间的量化关系，阐明焊点组织演化和抗蠕变性能退化及断裂失效的物理本质和微观机理，构建考虑有结构和组织不均匀性及耦合场作用的微凸点焊点蠕变本构模型，研究结果将为微凸点焊点制备工艺改进、可靠性与耐久性评价及失效分析提供理论参考。

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在电-热-力耦合场下的可靠性进行了系统研究。主要研究了结构和（或）组织不均匀微焊点中电流密度分布特征、非均匀分布电-热-力耦合场与微观组织演化间的交互作用、以及电-热-力耦合场下微焊点的拉伸、剪切、蠕变变形和断裂行为。本项目取得的重要研究结果和规律发现包括：基于焊点真实微观组织图片进行图像识别后导入有限元软件中构建模型，采用有限元2D模型可获得与3D模型相近的电流密度和温度梯度分布结果；相粗化对两相中平均电流密度和平均温度梯度影响甚微，相分离会使富Sn相中平均电流密度降低、富Bi相中平均电流密度上升、电阻线性增加；电流密度增大时Sn-58Bi焊点拉伸强度降低同时受富Sn相与富Bi相间的电流密度和温度梯度差异及相界面应变失配影响；Ag对SnBi焊点的强化作用在电流作用下被弱化主要源于被强化的微区电流密度、温度梯度和相界面应变失配；对比研究了不同焊盘组合的BGA结构Sn-3.0Ag-0.5Cu焊点在电流作用下的剪切性能，指明金属化层Cu-OSP和ENIG对焊点的剪切变形和断裂行为均影响较小，同时发现在较低温度下低密度电流有利于焊点剪切强度提升；量化剥离出电流的焦耳热效应和非热效应对焊点强度变化的影响，发现电流密度增大时，电流的非热效应对焊点强度的影响由强化作用变为弱化作用，而焦耳热始终弱化焊点的强度并在高温和高电流密度时成为主导因素；电流密度增大Sn-58Bi焊点的稳态蠕变速率不断增大、应力指数和蠕变激活能则不断减小，致使焊点的蠕变机制逐渐由体扩散主导变为位错管道扩散主导，并由此导出嵌入电流密度的蠕变本构方程。在焊点的拉伸、剪切和蠕变实验中，随电流密度增大，焊点的断裂位置均呈现出由钎料体内向界面转移的特征，呈现出韧脆转变。本项目所获得的研究结果和发展的研究方法推进了对结构和组织不均匀微焊点在接近真实服役条件下的性能和可靠性评估手段的基本认知，为微焊点制备工艺改进、可靠性和耐久性评价及失效预测提供理论参考。