温度梯度诱发的无铅微焊点组织演变、损伤机制及力学性能退化研究

Along with the progress towards miniaturization and multi-functionalization for electronic products, the characteristic dimension to interconnecting solder joints in electronic packaging field is more and more small, resulting in the current density through the solder joints and thermal gradient between the two sides of solder joints sharp increasing. Such phenomenon will trigger the electromigration(EM) and thermomigration(TM) effects in the solder joints, inducing the microstructure degradation and interfacial damage, so the reliability of the solder joints will be obviously decreased. Although now the research on EM effect has achieved many progresses in Flip-chip packaging, it is only in the incubation period for the research on single TM effect due to the difficulty of experimental design induced by thermal-electric couple effect. A bran-new experimental framework is advanced in this project. It is feasible that microstructure evolition and interfacial damage mechanism is researched for lead-free micro-soldering joints under the condition with TM effect without regard to EM effect. Meanwhile, the influence of single TM effect on the reliability of the solder joints is characterized by creep life when a external load is applied to it. Finally, the dynamic process is simulated by computer so that the intrinsical rule between microstructure degradation and mechanical property evolution can be explored. The research results achieved by the project will play an important role in understanding the physical essence of TM effect and forecasting the service life of solder joints as well as designing chip packaging framework.

随着电子产品向小型化、多功能化方向发展，电子封装互连焊点的特征尺寸越来越小，导致焊点经受的电流密度和焊点两端的温度梯度都急剧增加，从而引发焊点中的电迁移和热迁移效应，造成焊点组织劣化及结构完整性损伤，大大降低了焊点可靠性。在现阶段，虽然对电迁移的研究已经取得较大进展，但由于热电耦合导致实验设计的困难，对热迁移的研究尚处于起步阶段。本课题通过一种全新的实验方案设计，在不涉及电迁移要素作用的条件下，单独研究热迁移要素作用时倒装芯片互连无铅微焊点显微组织演变机理、界面结构损伤机制；同时耦合外加载荷，用蠕变寿命来定量表征热迁移现象对焊点可靠性的影响。最后结合对焊点在经受热迁移要素作用时焊点性能退化过程的计算机仿真模拟，来探寻焊点组织和性能动态演变的内在规律。本课题的研究结果，将为更加深刻理解倒装芯片互连焊点热迁移现象的物理本质、更加准确预测焊点服役寿命及更加合理设计芯片封装结构提供理论。

随着芯片集成度越来越高，电子封装互连焊点特征尺寸越来越小，（特别是3D IC封装时焊点特征尺寸已达10μm），导致焊点经受的电流密度和焊点两端的温度梯度都急剧增加，从而引发焊点中的电迁移效应(EM)和热迁移效应(TM)，造成焊点显微组织劣化及结构完整性损伤。本课题研究了不同特征尺寸的Cu/Solder/Cu、Ni/Solder/Ni和Ni/Solder/Cu焊点在温度梯度作用下的显微组织演变及对力学性能的影响，所得结果如下：.1）对于施加温度梯度时的Cu/Sn/Cu焊点，随着时间的增加焊点界面金属间化合物（简称：IMC）呈现不对称生长，即热端IMC溶解减薄、冷端生长增厚。当焊点高度较小时，焊点将完全转化成IMC。热迁移效应降低焊点显微硬度、剪切强度和蠕变寿命，焊点力学性能的退化程度大于时效。.2）对于Ni/Sn/Ni焊点，界面IMC在冷端和热端都增厚，但冷端增厚速率大于热端。热迁移使焊点剪切强度缓慢降低，蠕变寿命比时效时低。.3）对于Ni为热端的Ni/Sn/Cu焊点，热端和冷端的界面IMC都缓慢增厚，但冷端的增厚速率大于热端。由于焊点内存在大量的β-Sn，焊点的蠕变寿命和等温蠕变时相当；Ni为冷端时，热端的Cu大量溶解、并迁移到冷端，致使冷端IMC快速生长，此时焊点的蠕变寿命显著降低。.4）通过模拟仿真，对实验结果进行了验证，实验结果和模拟结果相符。.基于以上实验结果，探明了热迁移对焊点显微组织损伤及力学性能退火的机理：1）热端Cu溶解机制：Cu能在β-Sn中快速从热端迁移到冷端，导致界面IMC的不对称生长；2）热端Ni抑制机制：由于Ni的迁移速率大大小于Cu，可抑制IMC不对称生长；3）界面损伤机制：当冷端的Cu6Sn5和热端的Cu3Sn在热端相遇时，将弱化界面结合力；4）β-Sn损伤机制：使焊点热端β-Sn空位浓度增加，过饱和的空位析出将形成微空洞和微裂纹；5）β-Sn粗化机制：热端β-Sn晶粒尺寸大于冷端，导致焊点组织不均匀。所得研究结果将为准确理解互连焊点热迁移现象的本质、合理设计芯片封装结构提供理论依据。