倒装焊及相关问题的研究

【作者】 张群；

【导师】 谢晓明；

【作者基本信息】 中国科学院上海冶金研究所 ， 材料物理与化学， 2001， 博士

【摘要】 随着微电子产业的迅猛发展，半导体技术微细加工特征尺寸减小，晶片尺寸加大，IC芯片I/O端口数剧增，相应的芯片封装技术向高密度、高可靠性和低成本方向发展。倒装焊接技术因为具有近乎理想的封装密度和优异的高频性能，极有可能成为未来微电子封装的主流。国外对倒装焊先进封装的研究较广泛，涉及结构、材料、工艺和可靠性等等。而国内系统性的研究开展不多，本课题组对倒装焊先进封装进行了实验和数值模拟两方面的研究，旨在填补国内在这方面的不足，为例装焊在国内的大规模应用提供技术积累。 无损检测是微电子封装可靠性研究领域的重要手段，本实验过程中使用高频（230MHz）声学显微镜进行无损检测。结果发现，声学检测不仅对倒装焊底充胶分层敏感，而且随着实验过程的进行，相应的C-SAM图象焊点衬度变化直接与焊点的热疲劳裂纹萌生与生长程度相关。因此，在可靠性实验过程中，观察C-SAM图象中焊点衬度变化，可以在不破坏样品的情况下，定性探测到焊点热疲劳损伤程度，从而获得倒装焊可靠性研究中完整、准确和细致的信息。 使用底充胶可以明显提高倒装焊SnPb焊点的热疲劳寿命，但底充胶分层会影响焊点的可靠性。对不同参数底充胶试样通过温度循环实验并结合二维有限元模拟结果，研究了底充胶分层和SnPb焊点可靠性的联系。研究表明，底充胶分层易发生在芯片/底充胶界面边缘处；当底充胶与芯片粘合强度较弱时，底充胶的早期分层是焊点中裂纹萌生扩展从而导致失效的直接原因；当底充胶与芯片粘合强度较强时，分层可以削弱底充胶对焊点的机械耦合作用，从而影响焊点的热循环寿命，此时，焊点热疲劳裂纹是焊点失效的直接原因。另外，文中针对降低底充胶分层的发生进行了工艺参数和过程的优化。 通过对不同尺寸倒装焊SnPb焊点进行热循环实验，结合三维全局有限元模拟的结果，研究SnPb焊点热疲劳失效机制。结果发现，充胶后焊点内塑性应变范围减小近一个量级，从而显著降低焊点的疲劳损伤；由于底充胶改变了SnPb焊点应力应变分布，使得充胶前后焊点裂纹位置发生改变；充胶后焊点高度对SnPb焊点热疲劳寿命的影响变得不明显。热循环实验后，富Pb相和富Sn相都发生非均匀组织粗化，SnPb焊点热疲劳裂纹萌生于粗化区域。富Sn、富Pb相的非均匀粗化趋势与模拟给出的剪切应变轴向分布相一致。 ＠B 最后，通过高温高湿实验进行了湿度对倒装焊封装可靠性影响的探索性研究。研究结果发现，倒装焊焊点有一定抵御潮热环境的能力。实验过程中水汽主要通过基板背面渗入底充胶材料，引起底充胶与芯片界面发生分层，并且发现由于吸水导致的肿胀效应可引起分层区域发生动态复合现象。 本论文由于全程监测了可靠性实验过程，对倒装焊可靠性进行了综合性的研究，同时高频声学显微镜为研究过程提供了准确信息，对焊点裂纹萌生、生长及底充胶分层、传播等进行了精细的研究，从而对于倒装焊底充胶分层与SnPb焊点失效的相关性有了清晰的认识。本论文工作同时为倒装焊及相关微电子封装可靠性的研究提供有效的实验手段和方法，具有重要科研应用价值。更多还原

【Abstract】 With the rapid development of microelectronic industries, the reduction of feature size, growth in wafer size will continue in semiconductor industry. There is a crucial need for IC packaging to move to higher density, higher performance and lower cost. Flip Chip technology, by offering almost ideal density and superior performance, and could become the mainstream technology in the future. There are numerous investigations overseas, focused on the structure, materials, processes and reliability, etc. However in China, few systematic investigations were reported. In this project, flip chip on board (FCOB) technology was studied both experimentally and theoretically. The effort aim is promoting research activities in the field. Also it is considered to be important for the coming mass production with flip chip technology in the future. Undestructive inspection is an important technique in the reliability study on the microelectronic packaging. The undestructive inspection of high frequency (230MHz) scanning acoustic microscopy (SAM) is applied in our experimental study. It can be seen that not only the SAM is highly sensitive on the underfill delamination, but also the fatigue crack initiation and growth is directly coincide to the contrast variation of solder joint in C-SAM images during reliability test. Therefore the degree of solder joint fatigue of FCOB can be qualitative detected through comparing the contrast variation of the C-SAM images without destruct the sample. So the rather precision information in reliability investigation on FCOB can be accumulated through SAM inspection. The thermal fatigue lifetime of SnPb solder joints can be greatly increased by applying underfihl encapsulant in FCOB. However, the underfill delamination could correspondingly effect the reliability of solder joints. By conducting thermal cycling test and two-dimensional finite element simulation on the FCOB with different underfihl materials, the relationship between the underfihl delamination and the SnPb solder joint reliability is investigated. The results show that the underfihl delamination initiates at the edge of chip/underfill interface. If an underfihl did not have a good adherence to the chip, the fatigue crack initiation and propagation in the solder joints is caused directly by the incipient underfihl delamination. On the other hand, if an underftll had a good adherence to the chip, the dominant failure mode should be the fatigue crack in the solder joints. It is shown that the mechanical coupling effect of the underfihl could be weakened by the underfihl delamination, which should influence lifetimes of the solder joints. The processes are optimized to decrease the occurring of underfihl delamination. By conducting thermal cycling test and three-dimensional global finite element simulation, the thermal fatigue failure of SnPb solder joints of different size FCOB is investigated. The results show that plastic strain range in the solder joints is decreased to nearly an order of magnitude after underfihling, therefore degrades thermal fatigue failure of the solder joints subsequently. Since the thermal stresses and strains in the solder joints are redistributed due to the application of underfihl, the locations of thermal fatigue crack initiation and propagation in the solder joints are changed. Furthermore the height of the solder joint is found irrelevant to the thermal fatigue life of SnPb joints in Flip Chip. Heterogeneous coarsening is observed on both Sn-rich and Pb-rich phases after更多还原