无铅焊点微观组织的热疲劳损伤及纳米金属强化机理研究

At the moment, a major paradigm change, from 2D IC to 3D IC, is occurring in microelectronic industry. Joule heating produced by current is a serious challenge in 3D IC. In order to remove heat, a temperature gradient must exist in the packaging. Thermal stress would unavoidably imposed on the solder joints due to the mismatch of thermal expansion coefficients between solder and substrates. At the same time, most major electronic manufactures have stepped up their search for a substitute for tin-lead solder to protect human health and environment. Therefore, the thermal damages and reinforcement of lead free solder connections have become crucial issues. Thermal damages behaviors of interconnections with different microstructure will be investigated carefully in this investigation program. Thermal shear stress and tensile stress will load on the interconnections by special specimen design respectively. The damage mechanism of microstructure under accelerated temperature cycling will be elucidated. Metal nano-particles, as a strengthening phase, will be added in the lead-free solder to form a composite solder. The thickness of IMCs, resistance and total length of microcracks will be employed to evaluate the strengthening effect. The strengthening mechanism of mental nano-particles in composite solder will be illuminated according to the diffusion theory and dislocation theory. This investigation program will be of great importance in the reliabilities of electronic packaging. The main significances of this program are searching the method to suppressing and delaying the failure of lead-free solder joints and illuminating the strengthening mechanism, which could provide theoretical direction and experimental data for the improvement of microstructure and reliabilities of solder joints in electronic packaging industry.

当前微电子封装技术由二维平面互连发展到更高集成度的三维垂直立体集成互连，电流产生的焦耳热使焊点的可靠性面临严峻挑战，散热措施会在焊点间形成温度梯度，由于焊料与基板的热膨胀系数不同，焊点不可避免地承受热应力作用，同时，随着环保意识的增强，人们正在开发新型无铅焊料，无铅焊点微观组织的热损伤及强化机理成为一个重要的科学问题。本项目采用特定的实验设计对不同微结构无铅焊点的热剪切和热拉压损伤行为进行研究，通过加速热循环实验揭示热损伤机理；将纳米金属粒子作为强化相引入无铅焊料形成复合焊料，使用IMCs厚度、电阻和微裂纹总长度等参数评估强化效果，结合扩散理论和位错理论，阐明复合焊料中强化相对提高焊点可靠性的影响机制。本项目的研究旨在阐明热应力循环作用下焊点微观组织的损伤机理，寻求延缓和抑制各种失效模式的可能途径，并阐明其强化机制，能够为工业上改良焊点微观组织、提高焊点可靠性提供实验与理论指导。

出于健康和环保的考虑，当前微电子封装产业正在研发新型无铅焊料，本项目主要研究了无铅焊点微观组织的热损伤机理和纳米强化相的强化机制。加速热循环实验结果显示，焊点的微观组织对于热损伤机理具有重要影响。回流态焊点内界面化合物（IMC）层较薄，焊点的失效断裂主要是由焊料内β-Sn热膨胀系数的各向异性引起的，由β-Sn晶粒热膨胀系数的各向异性引发的晶界应力集中是热损伤的主要因素，微裂纹产生于β-Sn晶界处，并沿晶界扩展。经过时效老化后，焊点组织粗化，尤其是IMC层增厚，其体积分数增加，导致应变迅速积累，IMC层易发生脆性断裂，焊接界面的几何约束上升为热损伤的主要因素，IMC层厚度对焊点的可靠性起着关键性作用。我们通过向锡银铜焊料和锡铋焊料中添加纳米强化相，制备出了一系列复合无铅焊料。锡银铜无铅焊料实验结果显示，添加Mo纳米粒子能够细化焊料内部Ag3Sn化合物的尺寸，降低焊接界面IMC层的生长速度，根据IMC层平均厚度平方与时效时间的关系曲线，结合扩散理论计算了焊料内原子的扩散系数，结果表明Mo纳米粒子的加入提高了原子扩散激活能，降低了原子的扩散系数，从而抑制了焊接界面IMC层的生长。在锡铋无铅焊料内添加Mo纳米粒子，可以使焊料富Bi相的组织更加细化和均匀，EDS面扫实验结果显示，Mo纳米粒子主要分布于富Bi相，老化时效过程中复合焊料内Bi-Sn扩散层厚度降低，表明Mo纳米粒子对Bi原子的扩散也具有抑制作用。维氏硬度实验结果显示，添加Mo纳米粒子的锡铋复合无铅焊料合金硬度高于常规锡铋焊料合金。上述研究结果对于深入认识焊点热疲劳损伤的微观机理和纳米强化机制具有重要科学意义。